By my 20's it had pretty much gotten as big as it was going to and unfortunately, I think if anything I've lost size over the years. Mostly I think it's been because of weight gain as I was a skinny bastard up until my early thirties and thus didn't have the spare tire partial hiding its full length.
But I also just think aging changes can and did result in going in the opposite direction, though given the comments and feedback I've gotten from others both in virtual and real life, I guess it's not been too dramatic a reduction even if I seem to notice it and think so.