muscle weighs 3x more than fat.
Here's a visual:

Well, technically, it is impossible for muscle to "weigh" more than fat, the correct characterization is that muscle is denser than fat.
Kind of like the joke of which will hit the ground first if dropped from the top of a building, 1 ton of feathers, or 1 ton of rocks. The answer of course is that they will hit at the same time. Although it may take a heck of a lot more feathers to equal 1 ton than it will rocks, if they weigh the same, they will fall at the same rate.