I don't know why you would say he's not a big buck? I realize they get bigger, but that is a dandy trophy. I'd love to have that on my wall to go with the few whitetails I've got mounted. Heck, I'd love to have any mulie cause I've never got to hunt them....so that would be a once in a lifetime trophy for me.
I can't explain why he ran so far, but this is my take on it. If you were above and hit midways behind the shoulder and you obviously got a low exit wound, I would venture to say that you clipped the bottom 1/3 of the entrance lung and missed the heart. Or clipped the bottom of the lung and just grazed the heart. Deer can go amazing distances on one good lung, and on heart shots where no lung is hit, they can go 150 to 200 yards on pure adrenaline before piling up.
I killed a 155" 10pt. in 2008 with a rifle and hit right behind the shoulder and low with 130gr. Interbond, taking out the top of the heart and both lungs. The buck still went 80+ yards before piling up, and that just blew my mind how tuff they are.
You made the right call to give up the search and look again the next morning. Too many times people just have to keep looking in the dark and end up tromping all over what little sign there is and never find their deer. Had they just left it till daylight they might have recovered their animal. Congrats on a nice buck!