Plants are mostly made of cellulose, which is a bunch of sugar molecules linked into a strong chain.

Humans can't digest cellulose. For us, eating grass is just fiber, zero calories.

For herbivores like buffalo and cows, they have multiple stomachs and special enzymes and bacteria that let them break down cellulose into usable sugars. For herbivores, eating grass is like eating bread for humans. It's a form of carbs for them.

The other factor is large herbivores spend hours and hours a day eating like 50 lbs of grass. If you eat 10 loaves of bread a day you'd be 800 lbs in a few years too.