WarWagon

Odds are its fat. I'm the same way. My inner thighs aren't particularly strong, but the outer muscles on my thighs are somewhat stronger and larger than average. When you flex, the muscles tighten and bulge, which will pull the skin more towards the outside of your thigh, making things taught.

Edit: If you'd like to do firm up your thighs without hitting the gym, lay on your side and do leg raises. If you can't do them well at first with your leg extended, try bending it at the knee. Averett's idea with the basketball works well too, although some of those cheap bouncy balls you usually see in racks in the aisles at places like Wal Mart or Target work even better as you have a larger range of motion.