Ok? So do basketball, softball and tennis players. Are they trying to get big? No, they want to increase athleticism.
I worked with my cousin shoulder to shoulder, 3 months same diet same everything but he added double muscle mass because his father was too strong man without even working out. It's genetics and Foreman was genetic freak like Tyson was.
He did have 3 times better results than me, we tracked our routines and he was just getting bigger easier than me.
They were big man conpared to average joes. Find a man that was looking like Tyson in his early teens is hard.
Foreman trained by chopping woods and lifting calves plus he lifted weights while never wanting to work on heavybag unlike Tyson who didn't touch weights until prison.
Foreman didn't like cardio but gaining muscle and punching power. That's how he gained more mass and became more polished,stronger and experienced fight with pro fighting.