Cattle come in a variety of shapes and sizes and are bred for different traits.
Beef animals are stockier and thicker because they are primarily used for meat and dairy animals are used to produce milk. Although beef cattle also produce milk, they don’t produce it on the same scale as dairy cattle. Dairy cows focus their calories into milk production, whereas beef cattle convert their calories more into muscle, so what they look like is really dependent on what their job is, how they are bred as well as their genetic make up. (It’s like expecting a pug and a greyhound to be equal because they are both dogs.)