The actual number of muscle cells that your body has doesn't increase or decrease unless you are juicing. In which case the number can increase. But when you lose weight, the muscle and fat cells don't dissapear, they just shrink. So it makes sense to me that after you are done dieting, your muscle cells will want to get back to just about the same size as they were before. The same thing probably applies to fat cells as well. It makes it easier to see how muscle memory can occur once you realize that you are keeping the same muscle cells and not gaining new ones.