Ive gotten heat rash since I was a tiny kid. I remember being 5 or 6 and just laying in bed at night and being on fire, lol... I wasn't overweight as a kid at all.. It's just genetic for me. My dad gets it something awful, too.
I generally get it on the backs of my calves, just above my knees on the tops of my thighs, my chest, and my forearms. I've never associated it with being overweight. My mom was, and never had it, and my dad isnt and always has it, lol. It's just us.
The things that help me are: Ibuprofen (to reduce inflamation from the inside out!), oatmeal baths in cool-ish water, store brand aveeno oatmeal lotion (I usually get the Target one) and patience.