I'd agree when we talk about the size of spiders and stuff. But iguanas could have worked perfectly in the current habitat system (and if you have the mod, you can clearly see they do), but Frontier went for it most likely because it falls under the same category as the other animals as in "they don't move much in zoos". Because if size was the reason, the caiman would have been an exhibit box too. You could say that the caiman could have been an exhibit box animal too based on the premise of it not moving much either; but clearly they wanted it for the swimming/deepdiving.I agree fully with the statement on walkthroughs
however for the regular exhibit box I think there a 3 factors one is size there are just things too small to make work such as the spiders the second is just like the walkthroughs it provides ways of including animals like snakes with difficult locomotion styles to animate and 3 at the time of the base games creation it was a way of adding smaller animals that would have been annoying to work with in the current habitat system ie iguanas which easily move more then the tortoises but would be annoying to create when the smallest thing in the base game was probably also the tortoise and planet zoo is not built for small habitat animals at all.
Though, I heavily disagree with the notion that Planet Zoo was not build for small habitat animals, there's absolutely nothing indicating that. We've gotten several smaller animals, both in the base game and in the first year of DLC. Planet Zoo simply went for the bigger habitat animals first because those are the obvious animals you want to add for your general audience, because the same principles of attraction in real life zoos apply to zoo games as well. The majority of the people who go to a zoo go for the bigger animals, not the smaller ones.
It's no surprise that now that we're in the 4th year of development that we're getting smaller habitat animals, because those are the ones that the more "hard core" zoo audience also want in their game. Nothing fundamental was changed about the game that it now supports small habitat animals where it couldn't before, they always could, it's just a matter of releasing the right animals at the right time.