content: I'm not sure why you're being downvoted, but asking for a source is nice and well. Here you go. It makes for some nice reading, but to sum it up, the reason for the size difference is that muscle (more specifically skeletal muscle, biceps, abs, etc) is denser than fat. Skeletal muscle consists of long cords that are side by side, whereas fat is made up of adipose tissue which is very fluffy and takes up a lot more room. Also, according to LiveStrong, the actual size difference is that muscle takes up about 4/5 the space of fat.