They incorporate alot of scientific facts about organisms, not to mention their information is actually believable. Another thing, did you see how easily Grid tackled Celtic? -Hint hint-
Aliens are thing but heavily riddled with bones and protrusions, not to mention their massive head and tail, I am sure if you took those away they would weigh less, but including all of them Aliens outweigh preds by a good 20 - 30 lbs.
EDIT: SiL, I personally prefer the Resurrection aliens, they are more animalistic, creature-like version that should fight the preds, not the Bolaji alien.