It's not *just* because you're heavy. Whenever skin is rubbing together, it will get darker. Even skinny girls get darkened thighs if their thighs rub together. Not that I'm skinny, but I'm considered a healthy weight according to BMI, and my thighs still rub together and make dark spots on my upper-inner thigh.
I've heard rubbing lemon on your skin helps lighten it, but I have yet to try it. If you're going to give it a whirl, be very very careful with making sure the juice stays on your thigh and doesn't creep anywhere else, if ya know what I mean. The acidic lemon juice has potential to mess with the pH balance in your va-jay-jay.
"I regret working out today." -No one ever.