cows can be muscular through genetics; based of the parents and their shape, it can determine how the calf would turn out. another is the food you feed it, if you feed it the best food with lots of feed and grass/hay, it will help the outcome of the shape of the cow.
thats all i could really tel you bc my family has 19 cows and we breed them and sell them. i learn from my dad about them and that’s what he told me.
