Exercise helps because THC is stored as a metabolite in your fat cells. When you exercise you burn off the fat cells, and that releases the THC into your bloodstream and urine. You piss out the THC metabolites, so you get the THC out of your system.
For me, just exercising for a week and not smoking is enough to get me, as my friend says, STONED TO DA BONE!
And I'm 230 pounds, 6 foot nothing. I work out all the time though.