I think there is some thinking going on behind the make-up stying in ST.
As a rule, the more aggressive beings tend to have heavy ridged foreheads, which I have always presumed reflects thicker and heavier bones - the ridging would allow for overall thickness without excessive weight. For beings who like to hit each other a lot, it makes sense that they have developed something there to give some protection against frontal attacks. They thicker skull in other places is probably hidden behind their hair.
Those species who have developed a more Vulcan approach to life seem to have smoother heads, to better fit their larger brain capacity.
It is also significant that heavy forehead makeup tends to make someone look agressive, and smooth features the opposite, probably reflecting out own past developing from Neandethal through to Homo Sapiens.