What weighs more muscle or fat based on the same mass?

[Phillister]

What weighs more muscle or fat based on the same mass?

[carl669]

if you mean "same volume", then muscle weighs more.  (a pound of muscle sitting on a table would look smaller than a pound of fat).

[gerrymcd]

A pound of fat takes up about 4 times the volume (space) of muscle.  That is, a pound of muscle occupies less space than a pound of fat. So, if you swap 2lbs of fat for 2lbs of muscle you will weigh the same, but feel a bit thinner.

[MikeInSea]

The term "mass" is used to describe the "stuff" of an object or substance. It is usually measured by weight, given a constant force effect of gravity on an object on this planet. 

Using a measurement of weight, quantities muscle and fat of the same mass would weigh the same, assuming the effect due to gravity is the same. 

As others have stated, muscle is more dense than fat, and therefore muscle weighs more by volume than fat.