I agree with this, 100%. Please get it checked.
So, I really think you should see your doctor and have them evaluate. Testicular atrophy is something that can happen because of things like trauma, vascular issues, varicocele, etc. A testicle can eventually lose function. Evaluating it may prevent that. Small testicles, one smaller than the other are not problems. But the change is what I'd really want a doctor to weigh in on. It could be just getting older and nothing to worry about. But you want to make sure you know for sure. How old are you? That you have no change in drive is good. It really 'could' just be aging. But get checked out. https://www.healthline.com/health/testicular-atrophy#causes