diff --git a/.chloggen/deprecate_default_fetch_size.yaml b/.chloggen/deprecate_default_fetch_size.yaml
new file mode 100644
index 0000000000000..03bed1b6915dd
--- /dev/null
+++ b/.chloggen/deprecate_default_fetch_size.yaml
@@ -0,0 +1,30 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: deprecation
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/filelog)
+component: receiver/kafka
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`)`.
+note: "Deprecate `default_fetch_size` parameter for franz-go client"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [43104]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  The `default_fetch_size` parameter is now deprecated for the franz-go Kafka client and will only be used with the legacy Sarama client.
+  Users should configure `max_fetch_size` instead when using franz-go.
+  This deprecation is marked as of v0.142.0.
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]
diff --git a/.chloggen/fix_43104.yaml b/.chloggen/fix_43104.yaml
new file mode 100644
index 0000000000000..ba4679dd9b9b6
--- /dev/null
+++ b/.chloggen/fix_43104.yaml
@@ -0,0 +1,30 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. receiver/filelog)
+component: receiver/kafka
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Use `max_fetch_size` instead of `default_fetch_size` in franz-go client"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [43104]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  The franz-go Kafka consumer was incorrectly using `default_fetch_size` (a Sarama-specific setting) instead of `max_fetch_size` when configuring `kgo.FetchMaxBytes`.
+  This fix ensures the correct parameter is used and adds validation to prevent `max_fetch_size` from being less than `min_fetch_size`.
+  The default value for `max_fetch_size` has been changed from 0 (unlimited) to 1048576 (1 MiB) to maintain backward compatibility with the previous (incorrect) behavior.
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]
diff --git a/internal/kafka/franz_client.go b/internal/kafka/franz_client.go
index fc7cd48486a74..b0bd79737c08b 100644
--- a/internal/kafka/franz_client.go
+++ b/internal/kafka/franz_client.go
@@ -104,6 +104,8 @@ func NewFranzConsumerGroup(ctx context.Context, clientCfg configkafka.ClientConf
 	opts, err := commonOpts(ctx, clientCfg, logger, append([]kgo.Opt{
 		kgo.ConsumeTopics(topics...),
 		kgo.ConsumerGroup(consumerCfg.GroupID),
+		kgo.FetchMinBytes(consumerCfg.MinFetchSize),
+		kgo.FetchMaxBytes(consumerCfg.MaxFetchSize),
 	}, opts...)...)
 	if err != nil {
 		return nil, err
@@ -136,13 +138,7 @@ func NewFranzConsumerGroup(ctx context.Context, clientCfg configkafka.ClientConf
 		opts = append(opts, kgo.HeartbeatInterval(consumerCfg.HeartbeatInterval))
 	}
 
-	// Configure fetch sizes
-	if consumerCfg.MinFetchSize > 0 {
-		opts = append(opts, kgo.FetchMinBytes(consumerCfg.MinFetchSize))
-	}
-	if consumerCfg.DefaultFetchSize > 0 {
-		opts = append(opts, kgo.FetchMaxBytes(consumerCfg.DefaultFetchSize))
-	}
+	// Configure per-partition fetch size
 	if consumerCfg.MaxPartitionFetchSize > 0 {
 		opts = append(opts, kgo.FetchMaxPartitionBytes(consumerCfg.MaxPartitionFetchSize))
 	}
diff --git a/pkg/kafka/configkafka/config.go b/pkg/kafka/configkafka/config.go
index 16ae596b7018b..8a5ed599f7193 100644
--- a/pkg/kafka/configkafka/config.go
+++ b/pkg/kafka/configkafka/config.go
@@ -110,9 +110,10 @@ type ConsumerConfig struct {
 	MinFetchSize int32 `mapstructure:"min_fetch_size"`
 
 	// The default bytes per fetch from Kafka (default "1048576")
+	// Only used with Sarama client. Use MaxFetchSize for franz-go.
 	DefaultFetchSize int32 `mapstructure:"default_fetch_size"`
 
-	// The maximum bytes per fetch from Kafka (default "0", no limit)
+	// The maximum bytes per fetch from Kafka (default "1048576")
 	MaxFetchSize int32 `mapstructure:"max_fetch_size"`
 
 	// The maximum amount of time to wait for MinFetchSize bytes to be
@@ -145,7 +146,7 @@ func NewDefaultConsumerConfig() ConsumerConfig {
 			Interval: time.Second,
 		},
 		MinFetchSize:          1,
-		MaxFetchSize:          0,
+		MaxFetchSize:          1048576,
 		MaxFetchWait:          250 * time.Millisecond,
 		DefaultFetchSize:      1048576,
 		MaxPartitionFetchSize: 1048576,
@@ -174,6 +175,22 @@ func (c ConsumerConfig) Validate() error {
 			)
 		}
 	}
+
+	// Validate fetch size constraints
+	if c.MinFetchSize < 0 {
+		return fmt.Errorf("min_fetch_size (%d) must be non-negative", c.MinFetchSize)
+	}
+	if c.MaxFetchSize < 0 {
+		return fmt.Errorf("max_fetch_size (%d) must be non-negative", c.MaxFetchSize)
+	}
+	if c.MaxFetchSize < c.MinFetchSize {
+		return fmt.Errorf(
+			"max_fetch_size (%d) cannot be less than min_fetch_size (%d)",
+			c.MaxFetchSize,
+			c.MinFetchSize,
+		)
+	}
+
 	return nil
 }
 
diff --git a/pkg/kafka/configkafka/config_test.go b/pkg/kafka/configkafka/config_test.go
index 3bee81fe36e8e..3149f73a431ff 100644
--- a/pkg/kafka/configkafka/config_test.go
+++ b/pkg/kafka/configkafka/config_test.go
@@ -167,11 +167,34 @@ func TestConsumerConfig(t *testing.T) {
 				MaxPartitionFetchSize: 4096,
 			},
 		},
+		"zero_min_fetch_size": {
+			expected: ConsumerConfig{
+				SessionTimeout:    10 * time.Second,
+				HeartbeatInterval: 3 * time.Second,
+				GroupID:           "otel-collector",
+				InitialOffset:     "latest",
+				AutoCommit: AutoCommitConfig{
+					Enable:   true,
+					Interval: 1 * time.Second,
+				},
+				MinFetchSize:          0,
+				DefaultFetchSize:      1048576,
+				MaxFetchSize:          1048576,
+				MaxFetchWait:          250 * time.Millisecond,
+				MaxPartitionFetchSize: 1048576,
+			},
+		},
 
 		// Invalid configurations
 		"invalid_initial_offset": {
 			expectedErr: "initial_offset should be one of 'latest' or 'earliest'. configured value middle",
 		},
+		"invalid_fetch_size": {
+			expectedErr: "max_fetch_size (100) cannot be less than min_fetch_size (1000)",
+		},
+		"negative_min_fetch_size": {
+			expectedErr: "min_fetch_size (-100) must be non-negative",
+		},
 	})
 }
 
diff --git a/pkg/kafka/configkafka/testdata/consumer_config.yaml b/pkg/kafka/configkafka/testdata/consumer_config.yaml
index dc2a9d85127ab..938f02d15cafe 100644
--- a/pkg/kafka/configkafka/testdata/consumer_config.yaml
+++ b/pkg/kafka/configkafka/testdata/consumer_config.yaml
@@ -12,7 +12,16 @@ kafka/full:
   max_fetch_size: 4096
   max_fetch_wait: 1s
   max_partition_fetch_size: 4096
+kafka/zero_min_fetch_size:
+  min_fetch_size: 0
+  max_fetch_size: 1048576
 
 # Invalid configurations
 kafka/invalid_initial_offset:
   initial_offset: middle
+kafka/invalid_fetch_size:
+  min_fetch_size: 1000
+  max_fetch_size: 100
+kafka/negative_min_fetch_size:
+  min_fetch_size: -100
+  max_fetch_size: 1048576
diff --git a/receiver/kafkareceiver/README.md b/receiver/kafkareceiver/README.md
index 668ed2d117037..23cf34ee1f3cd 100644
--- a/receiver/kafkareceiver/README.md
+++ b/receiver/kafkareceiver/README.md
@@ -90,8 +90,8 @@ The following settings can be optionally configured:
   - If set to an empty string (or not set), the consumer is treated as a dynamic member. In this case, the consumer's partition assignments may change during rebalances.
   - Using a `group_instance_id` is useful for stateful consumers or when you need to ensure that a specific consumer instance is always assigned the same set of partitions.
 - `min_fetch_size` (default = `1`): The minimum number of message bytes to fetch in a request, defaults to 1 byte.
-- `default_fetch_size` (default = `1048576`): The default number of message bytes to fetch in a request, defaults to 1MB.
-- `max_fetch_size` (default = `0`): The maximum number of message bytes to fetch in a request, defaults to unlimited.
+- `default_fetch_size` (default = `1048576`): (Deprecated [v0.142.0]: only used with legacy Sarama client. Use `max_fetch_size` instead) The default number of message bytes to fetch in a request, defaults to 1MB.
+- `max_fetch_size` (default = `1048576`): The maximum number of message bytes to fetch in a request, defaults to 1MB. Must be greater than or equal to `min_fetch_size`.
 - `max_fetch_wait` (default = `250ms`): The maximum amount of time the broker should wait for `min_fetch_size` bytes to be available before returning anyway.
 - `max_partition_fetch_size` (default = `1048576`): The default number of message bytes to fetch in a request per partition, defaults to 1MB. If a single record batch is larger than this value, the broker will still return it to ensure the consumer can make progress. This setting only applies while using [`franz-go`](https://github.com/twmb/franz-go).
 - `tls`: see [TLS Configuration Settings](https://github.com/open-telemetry/opentelemetry-collector/blob/main/config/configtls/README.md) for the full set of available options.