Rob
Being overweight or not is not the cause of the problem. I had heat rash and chaffing problems since High School 30+ years ago. I was not overweight then, though some would consider me as overweight now at 6'2" 220 lbs.
Agreed. When I was a svelte and slinky 20 year old I got shipped to San Angelo TX by the Air Force and got heat rash over the whole area so bad it actually effected my sex life. And *that* was wearing loose, long shorts!

Even at my heaviest since, I have experienced nothing more than a few bumps.