If buffalos have got you thinking, I'm gonna blow your mind by telling you about elephants and brontosauri. Or even gorillas--these bulky beasts get super swole by eating plants.

You don't need meat to get big. Especially when you have a digestive system that is capable of getting nutrition from non-meat sources. Lots of animals have evolved different digestive systems than us. They get more out of the plants that they eat than we do.

(Heck, even humans don't need meat to get big. We just need protein--and this can come from many non-meat sources.)