Q: How much space does a pound of fat take up compared to a pound of muscle ? I?ve heard that fat takes up more space than muscle despite being lighter than muscle.

A: Fat is about 20% less dense than muscle, so it takes up about 20% more room than muscle. It's normal that the lighter substance (fat in this case) takes up more room than the heavier substance (muscle) -- just think how big a two-pound feather-stuffed pillow looks compared to same two pounds of, say, iron.