Chapter 19. Filtering Messages [1] states the following for AMQSize:

AMQSize

The size of a message in bytes. The value is an integer.

However AMQSize actually only refers to the encodeSize of the message. The encodeSize is the space the message takes up in the journal but not the actual size of the message.

See ENTMQBR-1778

[1] https://access.redhat.com/documentation/en-us/red_hat_amq/7.1/html-single/using_amq_broker/index#filters