Fat guys can be in good condition.
You know what's harder than throwing 70 pitches in 2 1/2 hours? Throwing 70 punches in 15 minutes.
David Wells didn't struggle. Bartolo Colon. I could run off a ton of football players. Fat guys don't run marathons because it's constant movement. Baseball is not constant movement. A pitcher exerts himself for 15-20 times over 10 minutes. Then gets to rest. It's not the condition of the body. It's the condition of one arm.