Okay... this IS the same as muscle weighing more than fat. Of course
of each weighs the same... BUT if as you said a pound of muscle takes up 1/3 the space then it DOES indeed weigh more than fat. A 6" square blob of muscle weighs MUCH more than does a 6" square blob of fat. So if you have the same number of pounds of muscle you will be much smaller than if you have the same number of pounds of fat.
Saying that "muscle weighs more than fat" is IMO NOT saying one weighs more than the other... it is saying that the same SIZE of one weighs more than the other, which is in fact true.
Same way a bowling ball weighs more than a balloon.
Muscle DOES weigh more than fat!!
Sure the same number of POUNDS of balloons will weigh the same but they sure won't be the same size!
Also... if cardio does not build muscle -- how did I develop such nice muscles in my calves and thighs when until recently ALL I was doing for exercise was aerobic walking?
Believe me they are THERE... I can SEE and FEEL them!!