It is almost an ill-conceived joke of biology.
I'm sure some of you do it. After a vein popping workout, you peek at a nearby mirror sneaking a discreet flex. More often than not, you even admire the pump as it stretches the fabric of your shirt. That tightness may even up your manliness by at least 10%. Then off you go to the locker room encountering several larger mirrors. And once you see the reflection of your pumped up bod you do a couple of full on front lateral poses or a double bicep curl or whatever pose you saw somewhere and admire the pump once again, that is, when you think no one is looking. You may even do poses and turns shirtless. More adoration of the pump comes as you polish your engorged muscles with suds in the shower, your delts, your biceps, chest, six pack but then you notice that your prized jewels have shrunk.
This is a condition that most men face. When most of your muscle groups are defying your shirt size after a satisfying bout of weights, your most valuable piece have shrunk. It can be embarrassing.
There is a simple explanation to this. As you all know, the penis is a chameleon of sorts - a very changeable organ depending on the surrounding circumstances such as company, activity, temperature, altitude and what not. This versatility in size is determined by the amount of blood that is inside it. If we're going to be scientific about it, there is a spongy tissue called the corpus cavernosum that inflates or deflates depending on the amount of blood it contains.
Now when exercising a specific muscle group, your blood is redirected into that muscle group. And when you pump up a bunch of muscles, most of your blood supply is directed towards those muscles. All other parts that need the blood less tend to shrink, including the penis. The same explanation is applicable to balls.
