I think it was brought up on one of the NECA figurines that Scar's clan comes from a mountainous region of Yaujta Prime which explained the size. Now I don't put that much stock in NECA's backstories, but I'm a bit hesitant to not now seeing as we now know Fox did have some semblance of an internal continuity though apparently it is affected by the reboot cutoff date.
According to Andrew Garza, author of the "Predator bible" internal document that FOX uses, the NECA backstories are "officially canon".
I mean, feel free to ignore them if you think they're dumb (I happen to think they're neat).
I like the idea of Predators coming in different shades and sizes. Whether its dread length or tusk size. Some things shouldn't change though. The Super-Predators looked way to different but it's explained that they are a sub-species of yautja. Sort of like Humans and Neanderthals.
Not just a sub-species, a sub-species that had been actively genetically altering themselves.