Hello Jon,
You are correct, 25 MB is the size limit for the CRES message.
Currently, there is a reported feature request to allow bigger size messages: https://bst.cloudapps.cisco.com/bugsearch/bug/CSCur54669.
You can subscribe to the notifications link to be the first to know whenever this feature is added to CRES.
Let me know if this helps.
Best.
José L. Dávila