I think it was a Kerio/GFI response to a Microsoft problem. Ages ago they were getting reports of Outlook users crashing when dealing with messages over 25MB. In fact, I've had Microsoft people explicitly tell me, via the Feedback Chat Window, that this, indeed, is an issue. Not sure about the "New Outlook" but it's irrelevant at this point since it doesn't support on-premise Exchange, which is how Kerio looks to it.
So, they (Kerio/GFI) put a hard 25MB limit on messages.
There is a setting in mail server.cfg but it's not officially sanctioned so it may make things buggy. (https://support.kerioconnect.gfi.com/hc/en-us/articles/360015197399-Large-attachment-emails-are-miss...