To test I created a queue on the C++ broker with a size limit of 512 bytes, and a policy of "ring"
First I sent 5 messages each of size 100 bytes.
Then I sent a 6th 100 byte message - as expected this dequeued one of the existing messages
next I sent a single message of size 400 bytes
This caused only a single existing message to be dequeued, leading to 5 messages being enqueued with a total depth of 900 bytes (as reported by qpid-tool). At this point I would have expected all but one of the existing messages to have been dequeued in order to make room for the 400 byte message
Finally I sent a single 1000 byte message. This message in itself should have been too big for the queue, however it was enqueued; and only one 100 byte message was dequeued - leaving a total byte depth of 1800 bytes being enqueued.