Can someone please say why if a man is healthy he has really low testerone? If his size of his testes dors not matter, then what does?

LOW TESTOSTERONE. Actually the size of testes is related to testosterone level. The smaller the testicles most of the time testosterone will be low. Majority of the testosterone is produced in the testicles. Having low testosterone doesn't mean tou are not healthy.
Low T. As men age their testosterone production diminishes but they are still healthy. Low testosterone can cause fatigue and low libido. Size of testes generally doesn't matter - serum testosterone can easily be checked with blood work. The treatment if low is testosterone replacement usually a topical gel.
Multifactorial. The body regulates testosterone production via the hypothalamic pituitary gonadal axis. The hypothalamus "scans" for the hormone in the blood. If there is not enough, it sends a signal to the pituitary which sends a signal to the testis to make more. Thus, even if you are healthy, any step in this process can go wrong, resulting in low t. Testis size is more dependent on sperm producing cells.