OK...so you know thats REALLY not how things work...but you cant explain how they do work?Its REALLY not how things in teh body work at all.

When i do a squat, my quads dont know where the weight is.
It could be on a bar on my shoulders.
It could be in a backpack on my back.
It might just be my bodyweight.
If my bodyweight is suddenly twice as much as usual, then my quads will have to work twice as hard to do a bodyweight squat. It's no different from doing a normal squat with a weight on my shoulders.