Paizo has confirmed that the Centaur and Minotaur ancestries in the upcoming Howl of the Wild book will be large size by default, with a heritage each that make them medium.
Now thematically, I think that makes sense, 5e’s ponytaurs always seemed like a cop-out. However mechanically, large size has the benifits that it allows you to reach more squares, even without additional reach, and makes it impossible for small creatures to grapple you. So I’m wondering if that’s powerful enough to require some kind of downside to balance it out.
What do ya’ll think?
Typically, large creatures have reach, yes. But it’s not a guarantee.
As for PCs riding each other, the GM guide recommends you do not allow it, but offers a solution to keep it balanced if you really want to.