message delivery failed with errorCode=RequestEntityTooLarge although only 100KB

I have a trigger that subscribes to changes in Cosmos DB and pushes the document to Azure Event Grid. I am getting FailedEvents.

From the diagnostic log, it is clear that the reason is PayloadTooLarge, although the message is only about 150KB. (Event Grid is supposed to support messages up to 1MB).

outcome=PayloadTooLarge, latencyInMs=86, id=8e10d942-42f5-435c-a416-49af181c936e, outputEventSystemId=e549e75e-dc5d-484f-a839-67ef9fd3210a, state=Filtered, deliveryTime=11/4/2024 7:51:11 AM, deliveryCount=0, probationCount=0, deliverySchema=EventGridEvent, trackedSystemTopicState=, eventSubscriptionDeliverySchema=EventGridEvent, outputEventFields=InputEvent| EventSubscriptionId| DeliveryTime| State| Id| LastHttpStatusCode| DeliverySchema| LastDeliveryAttemptTime| SystemId, outputEventFieldCount=, dedicatedQueueDeliveryQueueId=, requestExpiration=1/1/0001 12:00:00 AM, delivered=False id=eqpt-1111, inputEventSystemId=5d72a537-62ec-4bcd-b24a-3433a53cec68 publishTime=11/4/2024 7:51:11 AM, eventTime=11/4/2024 7:51:11 AM, eventType=update, deliveryTime=11/4/2024 7:51:11 AM, filteringState=FilteredWithRpc, inputSchema=EventGridEvent, publisher=XXX-EVENT-TOPIC.SOUTHEASTASIA-1.EVENTGRID.AZURE.NET, size=131852, subject=cosmos/update, inputEventFields=Id| PublishTime| SerializedBody| EventType| Topic| Subject| FilteringHashCode| SystemId| Publisher| FilteringTopic| TopicCategory| DataVersion| MetadataVersion| InputSchema| EventTime| FilteringPolicy, inputEventFieldCount=16, type=HttpPost, subType=AzureDataFactory, supportsBatching=False, aadIntegration=False, managedIdentityType=None, urlPath=https://xxx.svc.datafactory.azure.com:4443/triggerevent/XXTrigger/triggerCosmosDbChange, deliveryResponse=PayloadTooLarge, errorCode=RequestEntityTooLarge, HttpRequestMessage: httpVersion=1.1, HttpResponseMessage: HttpVersion=1.1, StatusCode=RequestEntityTooLarge(RequestEntityTooLarge), StatusDescription=Payload Too Large, IsRedirected=False, RedirectUrl=,

The code is in Azure Function and uses output binding to send messages to Azure Event Grid.

@app.event_grid_output(
    arg_name="outputEvent",
    topic_endpoint_uri="EventGridTopicEndpointUri",  # in application settings
    topic_key_setting="EventGridTopicKeySetting",  # in application settings
)                     
def cosmosdb_trigger1(
    docs: func.DocumentList, 
    outputEvent: func.Out[func.EventGridOutputEvent]
):
    logging.info('Python CosmosDB triggered.')
    if docs:
        for document in docs:
            json_obj = document.to_json()
            logging.info(f"Content: {json_obj[:1000]}")
            logging.info(f"len:{len(json_obj)}")
            event_message = func.EventGridOutputEvent(
                id= document['id'],
                data= json_obj,
                subject="cosmos/update",
                event_type="update",
                event_time=datetime.datetime.utcnow(),
                data_version="1.0"
            )
            outputEvent.set(event_message)
            logging.info(f'sent {event_message}')
    else:
        logging.info("No documents found in the trigger.")