I think you are confusing your actual testes with your overall package down there. Like your flaccid unit, your scrotum varies in size. Naturally, when hanging very loosely everything feels softer and your testes are very well defined hanging free from your body near the bottom of your sack. When your sack retracts (cold, exercise, etc.) everything feels much firmer or harder. I have had a soft, semi-turtled sack, and I have had my scrotum turtled to the point where my right testes wasn't visible at all. At my most or nearly most turtled stage everything feels firmer or harder. Lay down for an hour in a warm room wearing loose fitting clothing. When you stand up your testicles will be hanging at their maximum. Now go run two miles (should only take 10 minutes or so!!!), and the difference will be remarkable.

While I haven't measured my testes, my jeqing has increased the size of them, but this didn't occur overnight, and the size difference between when I started jelqing and now isn't startling or anything like that. And even though my scrotum hangs much more freely than it used to, it still varies in size depending on what is going on at any given moment.