Millions More

"why do my balls feel so much better after I cum? what's the physicality of it?"

It has to do with vasocongestion; the build up of fluid. During ejaculation (when you cum), your balls will lift up, and the cremaster muscles clamp down and squeeze the sperm out of the testicles. Afterward, your balls will drop back down. According to Health.com, the process works like this:

During the arousal phase, vasocongestion occurs and the testicles grow larger in size.

During climax/orgasm, which happens typically with ejaculation (in most men), a range of chemicals are released including norepinephrine, serotonin, oxytocin, vasopressin, nitric oxide and prolactin.

After the act, the blood from the pelvic region is released. Contraction of tensed muscles takes place. The testicles in the scrotum slowly recede and fall back into the sack. Morphine-like chemicals such as endorphins are released which can relax the body of the man.

They'll feel lighter to you. Like some men, you may think you've "emptied" them.

But that's not exactly the case because testicles produce about a million swimmers per minute. That means, by the time that boner recedes, 

you've already produced millions more

for the next go-round.

Ain't balls amazing?