Well technically it's not the mass, but the fact that a heavier body is
less effected by air resistance given the equation Force = Mass x
Acceleration.  Acceleration due to gravity is always some vector of 9.8m/s
(assuming you're not dropping off a cliff).  Of course, we don't often
mountain bike in a vacuum...and a good thing.  Otherwise the 10 year olds
I've raced with would lap me 3 times instead of just twice :-)