WRONG!

Pay attention to all the words when thinking of things from High School Physics.

IN A VACUUM all abjects fall at the same speed.

But on a cycle we are not in a vacuum, in fact on a road bike over 80% of the work you do at speeds of 20MPH is overcoming aerodynamic drag. Now look at an object, rider sphere whatever. The cross section goes up as the square of the height (diameter) the mass goes up as the cube. Result bigger riders decent faster. In the more real world there is more than just scale . Take a nice big beer belly. It has no impact at all on the cross section (provided it is a perfect beer belly and only sticks out forward) thus it contributes even more to the downhill speed.

You will not find formulas as position counts as does the bike. But a nationally ranked woman rider in excellent aero position will decent Mullholland Drive at the same speed as a very average 220lb rider sitting up. (Experimental results because the club regroup point is at the top of the climb).