Have you ever compared the skeletal frame size of a human to a cow? If you have, you can pretty much answer your own question as to why cows and bulls, no matter how muscular they are will always be bigger and stronger than a human being. And yes, it's all down to genetics. A human's DNA is different than a bovine's, which dictate how big and how much muscularity there will be for each species.
But of course it isn't all about genetics in between species, but within species. Just like in cattle, there are different families of humans that are more prone to having men with more musculature versus other families. A particular family or line in cattle will determine how big or muscular a bull calf will get when he reaches maturity. This is more obvious in those breeds that have grown popular over the years and have been raised and selected according to what a producer wants from such a breed. These popular breeds include Hereford and Angus cattle. It's also obvious in breeds that have been selected for producing a large amount of muscle over a short period of time, like that in Belgian Blues and Piedmontese. A Belgian Blue's musculature didn't come from working out in the gym, it came from a mutation in the muscling gene called myostatin.
As far as grass is concerned, it is only low in protein when it has reached maturity and has become dormant and is no longer growing, and because us humans do not have the proper digestive tract to efficiently digest grass enough to gain on it like cattle. A human's digestive tract is designed for eating meat and easy-to-digest plant matter like berries and nuts. A cow's digestive tract is well-built for digesting roughage like grass and legumes. Just like us humans, cattle need protein to grow and maintain body function, and have a good body condition, especially for a bull that has just come out of the breeding season, and for a cow after calving and going into milk production. Too little protein in a bovine's diet can be just as bad is it can for a human.
But enough of that. As to your question, we could simplify it down to just one word: GENETICS. :).***:D
They do eat a lot of grass/hay, sometimes they get fortified foods from the cattle raiser, but it is mostly due to their genes. Breeding is a very important aspect to creating a good herd.
Um, because they are cows and bulls.
Grass is low in protein and they don't really eat much else. I train like crazy and have a huge protein intake and I know that a bull would put me to shame in a weights room.
