How does the plant eating animals manage to grow so big?

Question

How can animals such as horse, cow or elephant, rhino, etc., grow so big by grazing and eating plants?

Even if they eat a lot of grass, are there enough nutrients in the grass? how do they get enough calories to maintain their size? where do they get protein to grow big muscles like horses and buffaloes, or even deer?

if animals can produce muscles by eating grass, how come meat eaters need protein to get big?

How does this work?

Answer 1

based on the genetic make up of that organism