Groups administrator FAQ says (emphasis mine):
Yes, the maximum size limit for messages sent to a group is 25 MB, including attachments (the normal Gmail limit). Group owners or managers can edit specific groups to set lower size limits. The default size for a message sent in Google Groups is 25 MB.
A moment ago I've tried to send a message to my own group, having four lines of plain text and a total of two attachments (PDF files) -- 13 009 kB + 3 427 kB (16 MB according to Windows). I was blocked from doing so with an error message that total size of message exceeds group limit.
What am I missing? Where is the remaining 9+ MB to really exceed mentioned group limit?
(I'm group creator / owner / admin and for sure I haven't set lower limit, simply due to the fact, that I don't even know, where such limit can be configured in group management panel)
Some side-notes and research effects:
1. An answer to this question says something about 4 MB limits per file, which is something totally new for me. Is this still an active limit (question asked in 2012, but answered this year) as I don't find even a trace of 4 MB limit in Groups administrator FAQ?
This would fit to my case as one of the attachments has 13 MB of size. But, again, I don't know, if that 4-MB-per-file limit really exists as this is something new to me.
2. Four years old comment to the very same question says that I can use an URL like this:
However, after using it (with replaced correct group name) I can't find any size-limits-related setting in neither this or any other section of my groups configuration.