For the model overhead to become significant enough to eat into the gains, the file size would need to be fairly small, right? I assumed nobody would use this for compressing anything below 100 MB.

I tested with 100 MB files because anything larger takes a long time to evaluate. The actual target was at least 1 GB, and in that case I would use a 100 MB model (Shannon entropy rules).

I also tried it on a 100 MB Photoshop file and was able to compress it down to 45 MB, whereas ZIP could only get it down to 60 MB. So yeah still not losing gains.