message.max.bytes and mirrormaker tool

I've tried to run mirrormaker tools in async mode and I get
WARN Produce request with correlation id 263 failed due to
[benchmark1,30]: kafka.common.MessageSizeTooLargeException
(kafka.producer.async.DefaultEventHandler)

I don't get error in sync mode. My message.max.bytes is default
(1000000). As I understand the difference between sync and async mode
is that async collects batches before sending. Does it mean that batch
should be less than message.max.bytes, or any single message within
batch should be less than that.

Thanks.