diff --git a/swh/journal/tests/test_kafka_writer.py b/swh/journal/tests/test_kafka_writer.py
--- a/swh/journal/tests/test_kafka_writer.py
+++ b/swh/journal/tests/test_kafka_writer.py
@@ -89,7 +89,6 @@
         brokers=[f"localhost:{kafka_server[1]}"],
         client_id="kafka_writer",
         prefix=kafka_prefix,
-        producer_config={"message.max.bytes": 100000000,},
     )
 
     expected_messages = 0
@@ -119,7 +118,6 @@
         "brokers": ["localhost:%d" % kafka_server[1]],
         "client_id": "kafka_writer",
         "prefix": kafka_prefix,
-        "producer_config": {"message.max.bytes": 100000000,},
     }
     storage_config = {
         "cls": "pipeline",
diff --git a/swh/journal/writer/kafka.py b/swh/journal/writer/kafka.py
--- a/swh/journal/writer/kafka.py
+++ b/swh/journal/writer/kafka.py
@@ -58,6 +58,12 @@
         if not producer_config:
             producer_config = {}
 
+        if "message.max.bytes" not in producer_config:
+            producer_config = {
+                "message.max.bytes": 100 * 1024 * 1024,
+                **producer_config,
+            }
+
         self.producer = Producer(
             {
                 "bootstrap.servers": ",".join(brokers),