You can't really judge the look of the Predators just by these set photos. I mean, thanks to the miracle of filmmaking, you make something that looks unimpressive in real life look absolutely terrifying on the screen. Just look at Jaws and the original Alien as examples. However, maybe they're supposed to be smaller than normal? Maybe these are child Predators too young to go on the hunt? Maybe this world is sort of a "play pen" to practice?