As a fellow fatty I asked this question when I went to Crawley a few weeks ago. Apparently it actually makes no difference. Although the weight slows you down up ramps etc you actually end up with the greater traction out of corners etc which means the weight differences generally make no odds. I was disappointed by this too considering my skinny mate had just beaten my fastest laptime by less than 0.1 seconds!
I find this difficult to believe for a multitude of reasons - mainly because I need a feasible reason or five, why my friend who is a couple of stone lighter than me, is considerably faster
