That's still about lifting weights, where size is an important factor in absolute strength, obviously the heavier weight classes tend to have taller guys with bigger frames and bigger muscles. Lean body mass is still what drives the weights up, not the fat.
... Even in olympic weightlifting, you have to propel your own bodyweight, anything that doesn't help flex the weight up is an added burden. You can also keep the bar closer to the body when leaner, so that would make a big difference with al things being equal.