The alien queen was captured by the predators and was being fed chemicals into her body in order to speed up her reproductive process, this might also explain the difference in the aliens (theoretical.
The queen was totally remolded by designers for AVP and was totally anamatronic, alowing her to be more streamlined - the queen in aliens and resurrection had two men inside of the mold controlling her limb actions which is why the queen looked wider in appearance - as far as the bilogical side of things go, i'm not up to date on why the queen was different in the AVP movie, i'd go with the fact that perhaps (like humans) no two aliens are alike and have small charicteristics that seperate them from the others, it's possible that the queen came from a larger host which would explain it's increase in size.
Damn i'm good.
www.thorndesigns.netdarkness of acheron