Gravity generally, for the sake of discussion, affects all bodies the same ... assumng both subjects are in the same place on the planet . http://en.wikipedia.org/wiki/Standard_gravity
Acceleration due to gravity in free fall is 32ft per second squared, ignoring other factors for simplicity sake. So while gravity affects both riders with the same force, the one with the greater mass ends up in the higher end of the acceleration and terminal velocity scale. Further complicate things with drag coefficients and the like you end up something like this.

http://extremesportsphysics.blogspot...nd-beyond.html