Normal testicular size is measured in volume: 12-25ml is normal. I don't understand imperial measures, so you'd have to convert that yourself, but I think it equates to normal being about 1.5 to 2 inches in longest diameter.
Which puts me in the above normal range: mine are just over two inches wide, and just under three inches long. I hadn't realised they were large until seeing an orchidometer for the first time during med school, and noting that the largest size pearl was about half the size of one of my testicles. I don't have Fragile X syndrome though.
The testicles produce testosterone, so in fact the causal link between larger testicles and greater testosterone levels probably goes the other way to what people have suggested so far: large testicles produce more testosterone. Testicular size is almost certainly 95% genetic, but I doubt there are any studies on this. If you're creating a mental database, I went through puberty quite early, and emphatically, you can draw your own conclusions from that.
Frankly, having large balls is mostly a nuisance. I mean, it's vaguely reassuring to my sense of masculinity, but people generally tend to focus much more on penis size than testicular size when congratulating one. Fortunately for me, my whole genital ensemble seems to be well-proportioned, ie the large balls don't stick out or look ridiculous.
All good so far - but one major disadvantage is finding underwear that fits. Freeballing gets uncomfortable because of the weight, and the flopping around, and the propensity of the testicles to fall in between your legs if you sit down, etc. Even underwear with a large supportive pouch can sometimes be trouble: I've found that most underwear like that carries two "ribbings" on either side of the pouch, and my balls are wide enough that during the course of a day, one of them will end up pushing its edge under the ribbing, which then cuts into it painfully, and gradually becomes worse... And this problem isn't apparent until you've worn the underwear for hours. I've given up going through the awkwardness of asking for underwear with extra front room, because even stuff that comes recommended rarely seems to work. It doesn't help that I'm a shower rather than a grower.
With that off my chest, I will say: larger testicles probably carry a very slightly higher risk of testicular cancer as more volume = more cells = more chances for things to go wrong.
And, a study in the UK I think it was, showed that males with larger testicles are more likely to cheat on their partners. There's some link between promiscuity and testicle size - monogamous primate species tend towards smaller testicles, while polygamous ones, eg chimps, tend towards larger relative size.
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