i had one in XL a while ago, and fit me pretty well with a bit of room to spare. i'm 5'-11" and not exactly in the best of shape (unless you consider round a shape) so i couldn't possibly fit into a Large. however, if you a bit bigger than me even the XL will be a tight fit. my guess is because these vests are designed that way to carry a heavy pack on you back, so you wouldn't want any slack.
anyway, about the zipper in the back, it's on the inside lining of the vest. i was wondering what that pocket was for as well.