I am having a similar problem. My legs are big, but my torso area is small. I'm finding that the tops of my shorts shift around a lot and can cause discomfort. I'm forced to 're-adjust' the upper half (including the chamois) numerous times on my longer rides.

I keep trying different brands of shorts, and I have yet to find a pair that fits my big legs but that isn't too loose on top. As it is, I compromise and let the leg gripper squish my leg fat more than I'd like to get a waist that isn't super huge. Annoying. I'm also on South Beach and the problem is getting worse as the first place I lose weight is in my belly.

I have no solutions. Just commisuration!