Neha, thanks for the tip. Useful util! My problem was simple -- I missed one of the size field changes in the producer, which led to a completely wrong size field.
Am I correct in believing that the broker doesn't sanity check the message size field against the received data? In this case, the size of 200k+ makes no sense, and in particular, the broker isn't recording 200k+ worth of data, so it clearly understand at some level that the payload is shorter. ben