First of all, why do some mammals have such large testes?
It seems that it's all about sperm competition.
As in the bonobo (Pan paniscus), which is polygamous and promiscuous, the more sperm you produce, the more chance you have of fathering the next generation.
Next time the Winter Solstice comes around in your neck of the woods, try to imagine the beginning of an amazing transformation in the birds around you.
As soon as the photoperiod (daylight hours) begins to increase, male bird testes begin to grow and enlarge and the hormones flow!
At the same time, courtship behaviour, territory establishment, banishment of last year's youngsters and nest building begins.
In about 28 days after the Winter Solstice the testes of the Eurasian magpie (Pica pica) can both increase in size from a volume of about 10 mm3 to a whopping 200 mm3.
And, as soon as the mating game is over by mid-summer, the testes would have regressed to their smaller size. It's all about selective pressure to reduce the energetic cost of flight, which is very expensive.
In human terms that would mean each testicle would end up going from a volume of about 20 cm3 to 400cm3 (or the size of a small human head!). Or, from 15 to 300 grams, giving a combined weight of 600 grams in the mating season (if there were one).
Sport would be cancelled!
If you are still interested in us primates, see if you can work this one out:
Read more about genitals!