I'd rather not be micromanaged on every nuance and I'm sure they don't want that headache either. The DM can flat out tell you nothing outside of the normal spell. This is really a non-issue in my opinion. Do you really know a DM who would allow a giant ape? That sounds like a slippery slope if I've ever heard one.
I think the idea is that the clear "upgraded" mount for a Paladin are the Pegasus/Nightmare/Hippogriff/etc. But I can understand restricting them to the ones particular in the spell because the Paladin is pretty beastly already, and letting them have even stronger mounts would be a bit much.
But, as it, the rules encourage a Paladin to mount their war horse, cast a buff, then dismount and control the mount from the ground. Pretty silly to me.