bergstromtori,
Shorter people have, an average, smaller kidneys than taller people. And, younger people [40 is younger] have kidneys with more reserve than older people [I am 70, was 68 when I donated]. So, your kidney was not "tiny," but probably was smaller that if you have been 6'1. BUT, your age is helping overcome that.
I won't tell you not to "feel like a failure on this front" -- your feelings are your feelings, and we seldom can control our feelings. BUT, realistically, neither you nor your former, now your recipient's, kidney are failures. YOU took a risk that most people do not, to help another person. THAT KIDNEY, even if not as "perfect" as may be desired by the docs, is about
5 times more effective than dialysis -- and is also better than a deceased donor's kidney. The recipient is lucky indeed to have your kidney -- and I make a LARGE bet is
not thinking his new/used kidney is a "failure" at all.
Bill