Since the mid-to-late 1950s, when he was in his 20s, Botero stumbled on his trademark subject matter of grossly fat people. That takes in nearly his entire career. But for a few years his style was different, though quickly evolving into what made him wealthy.
I found a few examples of the early Botero to show you:
Typical Botero painting to set the scene.
Most of these folks are scrawny.
A normal-size person.
Now we see some unnaturally chubby horses.
A heavier human, but within normal bounds.
I almost think the overweight Diego Rivera should have been the artist here and not the normal-size Botero.