I was aware that a limit of 25 MB per single message posted to Google Groups exists, but I wasn't aware of any other limits and thus I was quite surprise to find out that Google Groups web interface block me from sending a message with just a two files, having 16 MB size in total.
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.
So, what's wrong?
The answer is quite simple. Even though Groups administrator FAQ says nothing about this, another limit of up to 4 MB per each attachment really exists, when posting to Google Groups.
Thus, example message posted to Google Groups with 10 files per up to 2 MB each will be successfully posted, because neither official (25 MB per message) nor unofficial (4 MB per file) limit is exceeded in this scenario.
While another example message with just 2 files but at least one having more than 4 MB or with as many files as you want having more than 25 MB total will not be posted to Google Groups.
This post is nearly an exact copy of my answer posted to Web Applications Stack Overflow.