I've had this problem many times before and Im not a huge guy either. For the most part- the sweat that causes this is from when it runs down your back and into your ass and then down your thighs. When I know I am going to be walking around a lot and its going to be hot out I roll up a paper towel and stick it in the top my my ass crack and have it up so that it collects the sweat from my back and is held up by my underwear/boxers. Its a little uncomfortable at first but it is way better than chaffing.
