It’s often claimed that a fat weighs more than muscle. Some even go so far as to claim that a pound of fat weighs more than a pound of muscle. There are even graphics produced which show huge differences in volume such as:
Or the much more ridiculous graphic:
What people really mean is that fat is less dense than muscle. Oil, for instance, floats on top of water. That is because oil is less dense than water.
On average, the density of fat is 0.9 g/mL. The density of muscle is 1.1 g/mL. Using the averages, 1 liter of muscle weighs 1.06 kg, or 2.3 lbs., while 1 liter of fat weighs .9 kg, or 1.98 lbs. (Source)
The difference is less than 20%.