That doesn't explain why the game actually worked at launch before the change (or multiple changes as I've read somewhere). So it's not really a 360 turn radius limitation kind of thing. Plus the animals can actually turn in place.
IIRC there were serious clipping issues at launch and problems with animals getting stuck in places where they couldn't turn around. I'm not entirely sure why it was changed again later, but my theory is that the boxing issue many people were experiencing was related to hitboxes as well, since the next increase seemed to happen around the time that was fixed.
All in all it seems like it's more of an AI limitation, at least the way I'm looking at it. It's all well and good to suggest that an animal should be able to pass through a narrow passageway from one large space to another, and then turn around in the second large space to go back through the narrow passageway, but if someone makes a narrow passageway that doesn't lead anywhere, can the animal get back out again?
In real life, sure, animals can and do reverse (often awkwardly) to remove themselves from tight spots, but is this a feature in the game? Is it something that can be easily implemented?
I definitely agree that the ability to use smaller doorways would be nice (especially for tiny animals like the capuchin, red panda, and pangolin, the latter of which can't even use the regular doorway building piece), but I'd be willing to bet there's some kind of good reason that they currently can't.