Although I hate ant-like explanations, I'm going to take Alien 3 Assembly cut as canon and accept royal facehugger as explanation.
And maybe Alien Runner, wasn't "runner" type because he was implanted inside an OX.
Maybe he was that way because his function was protecting the queen and being the first alien to serve her.
In ant colonies first ants mature faster but are smaller than next generations.
Runner chestbuster was fully formed, he has matured fast, but was smaller than typical alien.
However the question of who had brought the egg on Sulaco is still on the table.
And my bet is Sigourney Weaver did it in an effort to secure another Alien movie act.