If Greenskins were like their 40k counterparts as the older editions claim they were spores that hitched a ride on the Old Ones' ships and set up shop when they land, they'd easily overwhelm the entire Warhammer Fantasy world long ago even before Chaos come crashing in through the Polar Gates.
I mean, it does make sense when you remember that Orks grow with the quality and quantity of their enemy, in 40k, compared to Fantasy, the Quantity and Quality of the enemy is stupid high, as in you could blow up a star system and still get a fight that'll trump ya.
Whereas in fantasy, the Orcs have to deal with medieval warriors with much less numbers, thus limiting and lowering their growth and strength because of the low quantity and quality.
Effectively, the Fantasy Orks are just Orks undergoing insular dwarfism due to being just isolated on Fantasy's planet, and as such, are weaker and less then a normal Ork.
(To explain insular dwarfism, it's an process where an typically large animal, like say, an elephant, undergoes shrinkage in height, weight, mass, etc due to being limited to the range of an small island that usually wouldn't have been able to support their existence.
Something that the Warhammer Fantasy World can be considered when one remembers that 40k Orks usually travel the galaxy looking for fights and has more then enough krumping to last them a life time.)