So here's the background:
My player is making a fullblooded orc armored battlemage magus in fullplate. Who wields an orc skull ram. Not really your optimized scimitar dervish dance + intensified shocking grasp crit-fisher. But a really cool character concept nonetheless.
So if my player wants (they haven't decided) I will allow them to take a caypup with the mauler archetype. I realize technically improved familiars can't be maulers, but caypups can't speak anyway and have a 6 INT to begin with. I'm pretty sure that magus can take improved familiar feat if they have the familiar arcana.
I believe at 9th level the caypup would have 22 STR in battle form. Even with mauler's endurance and mage armor, it won't be nearly as tanky as an animal companion. Honestly it's not all that impressive, but fits the theme.
I'm less worried about this being overpowered as underpowered, but any feedback is appreciated. And yes, I know I'm bending the rules. Rule of cool and all that.
I've let my wizard put the mauler archetype on an improved familiar, and it didn't really affect anything since they didn't build around abusing it.
I don't think your ruling will break the game at all, but I would make sure no one else in the party is trying to build an animal companion focused character. They'll constantly be comparing their AC and class kit against what this Magus's familiar and class kit can do.
I bend/break the rules to do rule of cool stuff all the time, and 99% of the time it works out perfect, but it has become an issue before. So when we do rule of cool stuff, I have a conversation with the players in the campaign zero session that I reserve the right to modify/change things if it starts becoming an issue mid-campaign.
Personally, I don't think the caypup is going to break your game, but you should still have a conversation with your player.