My guess: I think the hair acts acts as a physical buffer from our bodies. When the hair is removed and the balls make more contact with our thighs/skin, they get warmer and have to hang down further to cool off.
It could also be an optical illusion because the hair is gone. Just as hairless balls are supposed to make the cock look bigger, the balls could also look like they're stretched down further than normal but, in reality, aren't.
It's easy to test. Put a warm washcloth on your balls for 5 minutes and then stand up or sit on the edge of a chair to let them hang and measure them. Then, let your hair grow back on your balls and do the warm washcloth thing again and remeasure. My guess is there shouldn't be much difference between the two.