I have noticed the same. My pt-111 came with two 10 round mags, but this was during the Clinton Gun Ban. Was temped to order a 12, but then realized that what I carried the 111 for, 10 rounds hopefully would be enough. And it if made the gun's profile larger (longer grip), I don't need it. If you get one, let us know how it looks. Your are right, if the exterial mag size is the same, go for the 12 rounder.
Wally