People saying a 5'6" guy couldn't kick the crap out of someone are being ridiculous. Someone that's trained to do it can absolutely kick the crap out of someone who's much larger, but untrained. You don't have to be the biggest and the strongest.
As for Cruise in the role... I haven't read the books and know nothing of Reacher, but I generally don't buy in to the author's physical description of the character unless its integral to the character. Playing the part is more about attitude, IMO.