Cows and other animals have been intentionally bred to become bigger.
If you look at medieval illustrations of shepherds and farmers [0], one thing that strikes you is just how small all the animals are. Even in relation to the medieval humans who were significantly shorter than us.
It had its advantages - for example, a leaner, smaller animal can walk long distances and won't get stuck in swampy ground. But it doesn't give you a lot of anything - hide, meat, milk... Nowadays, we have huge animals, which nevertheless have to be transported by trucks. No longer capable of walking 50 miles from the lowland to the mountains to graze.
