A message broker should be able to handle large messages (text/binary )  or arbitrary size. Message brokers are typically supposed to handle large quantities of small messages and in some cases there may be limits on the message size. 

To solve this problem the entire message payload can be stored in some exteranal service like redis  or database and the the message sent to the message broker will only have the reference to the  message hence this pattern is called reference based messaging pattern or claim check pattern.

Note that the pattern should be used only when the payload size is more to minimize latency.