I've noticed that if I wear a cock ring, my scrotum swells modestly, and people often comment on my sack. Although I've never done it, I've been told it is quite easy to enlarge the scrotum greatly by vacuum pumping. Not only do the testis enlarge a bit, but the sack fills with fluid drawn from the tissues, so it gets big and heavy. For those more extreme-minded, temporary scrotal inflation with saline is also fairly common, giving a huge sack, and silicone can be used for permenent results.
I've seen some people who present awsome bulges after using any of these techniques. I belive it's easier to get a lot of bulk by working on enlarging the scrotum than the penis itself. I've seen people fairly easily get their sack to small mellon size, despite having a smallish cock. This may be part of the reason why people do this. While a large penis has it's value, there's considerable recognition given to big balls, rather than the penis, in terms of being symbolic of strength or masculinity. Consider common phrases like "Do you have the balls to..." or "Stop being a wimp and show some real balls..." or "He's got a set on him...," etc. While a big dick is suggestive of sexual prowess, big balls seem to be more representative of masculinity in non-sexual ways, too.