If the combined weight of you & your bike is heavier than anyone else you ride with then you will go faster downhills. To some degree/at some/given point.
I know this cos I’m loads heavier than my mate (same bike weight or thereabouts) & I go faster than him without pedalling, generally.
Have you tried swapping bikes? My physics is a bit rusty. but shouldn’t all things fall at the same rate on Earth.. air resistance etc aside and being fat should increase resistance. I’m guessing your bike has less rolling resistance or it’s your technique (subtle pumping etc).
Yes. If you dropped two bikes off a cliff and one was 20lbs and the other was 35lbs, it would be resistance that dictates which one hits the ground first not weight. It’s possible that the lighter bike will hit the deck first if it falls in a manner that offers less air resistance.