So my husband has one testicle smaller than the other, it's like half the size. I'm sure that the other one is not too big because it looks normal. And this is not just a sudden thing, it happened like 3 years ago, maybe and I've been with him for 5years. We didn't even realized when it happened. It doesn't hurt or bother him. We've already had a daughter 4 months ago. Should we worry?

How does one tesitcle disappear and the remaining one shrink substantially?

I hope he's seen a Physician about this. And as a guy, I'd notice a testical missing in short order.

Is this a serious post?

Oh it's a serious post. But one didn't disappear. One is regular size and the other is half the size of the regular one. So he has both just not the same size.

He should still see a doctor about it.

Anytime there is a significant change in a person's body, they should see their physician.

Definitely he needs to see a physician.

He absolutely needs to see a doctor. The sooner the better. It is possible that this is something called tortion of the testicle, while painful for most people, I recently saw a patient who had no pain from this.