Oh dear god, Bob, that's the most asanine thing I've ever seen.
Which weighs more, feathers or bricks if they're not in equal quantities?
Muscle most certainly weighs more than fat. 5 lbs of muscle does not weigh more than 5 lbs of fat, admittedly, but that's not what anyone's saying.
Yes, people have found that they do tend to weigh more after a workout. Most of the explanations that I've found are water retention after a workout, and most say their weight is back to normal a few hours later.