The maximum blob size is related to the page size, but even then you can
store blobs slightly smaller than 4GB with page size 4096, larger page
sizes allow you to store even larger blobs (though some API functions
expect blobs of 4GB or less). The only possible reason I can think of
would be if the file is very big (lots of GBs), and you created the
database with a smaller page size than your Firebird 2.5 database.
I guess this could be an IBExpert limitation, but as I don't use
IBExpert myself, I'm not sure about that.