I think the hang-ups over size of sex characteristics are ridiculous. Unless someone is on a ridiculously extreme end of the spectrum, be it large or small, you're fine. Some people think that excessively large penises and excessively small vaginas are ideal. Well, if you took those two ideals and put them together, there are going to be some problems!
Besides regular growth from a child to adult I don't think it can really change all that drastically. Now if a man loses weight, it can make a pretty big difference depending on how much he loses and what his weight is. That thick area above the penis, the pubic bone has a lot of skin and fat there and when you lose weight it can kind of allow your peter to come out since the extra was in the way. Do I make sense? Lol. I rarely do, but this is true! If there's a significant amount of weight loss then it will appear longer. Not thicker, just longer.
Actually, there's exercises you can do that stimulate cell growth in the penis, making it longer/wider with time, same with breasts. My boyfriend has experienced noticeable growth over the past year and a half of him doing exercises, nothing ground shattering, but about half an inch of growth (he very regularly does them, it definitely takes commitment) and a noticeable bit of girth. Massaging breasts encourages them to grow slightly as well, it's the same concept as massaging a scar, stretching and moving flesh encourages cell growth. No pill can encourage growth in one area of the body like that, I'd be much more open to a cream or something that's directly applied, but so far I haven't seen any that cause more than temporary swelling.
This probably isn't what you meant but if a man gains weight it can make his penis shorter because the fat is covering/obscuring some of the shaft. I was surprised to learn on Dr Oz that this can make an inch or two of difference.