Why can two people of the same height and weight be two different sizes? Because one of them has more muscle. Muscle weighs more than fat (and burns more calories). The person that has more muscle is going to be smaller. Here is a photo showing the difference (in size) between 5 lbs. of fat and 5 lbs. of muscle:
Skinny vs Fit:
This is what fat looks like:
See before and after fitness photos for motivation!