Yes it is. It's HUGE!
I don't remember it it's ever legitimately explained as to why ADI made it so big (probably related to puppetry and articulation), but from a biological stand point it might be that the alien embryo will develop to a point that its host body can accommodate. Meaning that an ox has a much larger area of development in its chest than a human does, so an alien embryo is able to gestate a little longer to allow for further development. Basically the idea that some organisms can grow to match its environment. However, its probably more related to the fact that an ox is much larger than a human, its young are larger than human offspring, therefore an alien born of an ox would be larger than an alien born of a human.