Fact or Myth

It's not necessarily that it weighs more than fat, but 1lb of muscle will have a much smaller volume than 1lb of fat.

That's kind of a flawed question, and a flawed expression. Muscle is slightly denser than fight at about 1.06 kg/L. Adipose tissue (fat) has a density of 0.92 kg/L. Their densities are different, but it is often overblown in pictures showing a lb of muscle and a lb of fat. Their volumes for equal weights are not very different to be honest.

The more likely scenario is that you are either overestimating calories you burn from running, or underestimating your true calorie intake, or both. If you are not weighing the food you eat before you eat it, buy a scale and start doing so immediately. Be accurate, be honest, and be conservative in any estimations you must do.

lb for lb its the same. if you then look at the volume of the same weight - fat takes more space, muscle less. Have you been keeping track of your measurements? Do your clothes feel looser?

This can become a very complicated question. Worked muscle can retain water for the repair activity following exercise. Etc.