In the "Midgard Heroes Handbook" from the Kobold Press, there is a playable character race, the Centaur. Their size is rather unique – a Medium/Large hybrid, so to speak :
Size. Centaurs stand between 8 and 9 feet tall and weigh in excess of 1,000 pounds. Your size is Large.
Humanoid Torso. Although you are Large, you wield weapons and wear armor sized for a Medium creature, thanks to the proportions of your humanoid torso.
It is clear to me that, as a creature with a Large, equine lower part, a Midgard Centaur can be ridden by a Medium creature (provided they’re wearing a saddle), but as a creature with a Medium, humanoid upper part, they cannot wield oversized weaponry.
But what about grappling and shoving ? Are they considered Large, or Medium for such things ? I can see an argument for either interpretation – since they’re of Large category, but have Medium sized hands…