His friends were probably watching and laughing at you. Next time smack him back, he probably won't do it again and if his mates come out then leg it, but all the time you don't defend yourself you will remain a target and object of mirth among his peers.
Im sorry, but I have to disagree with this fully. Surely this method, presents just as much chance of being even more seriously hurt, as it does the other guy getting a beating?
Example one...Small kid comes up and hits you and runs away...a broken tooth is suffered, but you live to fight another day
Example two...Small kid comes up and hits you, as he starts to run you grab him and smack him back, he landing on the floor, whilst down there as you start to walk away, he takes a knife from his pocket, and stabs as you walk away...You dont get to live to fight any more days.
Yes there is example three where, he gets knocked down and stays down, but is it REALLY worth it, for the possible risk that it could entail, especially living in the world we do, where petty knife crime happens as often as it does? I dont believe it could be worth it...Id make do with one punch and my life anyday!
And also, it definitely isnt always about how big you are. One of my best friends is a monster, 6ft7, and built to match it, shaved head and a deep Geordie accent. You would not walk past him on a street alone late at night....unless you know him....And when you do, you know he would start or retaliate in a fight, never has and never would. His size gets him into trouble, as people seem to think taking on the biggest guy in the room gives them some praise, but he would never think that his size would give him an advantage, unless him life depended on it, in which case im sure he could if he needed to.