It's not gravity - gravity accelerates everything at the same rate, regardless of weight. Your increased density gives you more resistance to drag - thus, in a way, making you more aero.

I'm big. 210-215ish @ 6'3" so potential energy is my friend.
I crest the hill as far back in the saddle as I can comfortably sit, providing me with a slightly longer descent than those around me that are forward hugging their headsets. Longer descent w/ more mass = faster descent. (HA!)
I get aero by grabbing the clip-on bars and tucking my elbows into my chest/stomach. Usually still seated but occassionally "standing" on one pedal (assuming I cannot pedal any faster or I want/need a break) with my backside as far up & back as possible.
I have used a "lean really far forward" approach but found it to be a bit less stable, although possibly faster.
+++

That guy is really far forward. When I do it, I am much closer to the saddle and I am a bit lower. Regardless of position, I found it works better than my Cancellara-like tuck.

Ok. I'll tell my gut that next time we're both rocketing down the hill.

i used to do this then once i hit a big pothole and the jolt caused my hands to slip and i wobbled a bit and then made a bad smell. after that i either hold the drops or have my hands on top of the hoods but get down low.

Now, I'm going to look all sorts of crazy on descents trying to figure out which one is faster... :\