Today's entry falls into the "why are these scary?" category. I think, when I tried to do this last year, that I was quickly falling into a rut trying to explain why certain things were scary. Fear comes from being unfamiliar with something and while each flavor may be distinct, they all spring from the same common source- the unknown.
Giant Animals fall into the "nature gone wrong" category of horror. There are only a few predators out there with the ability to scare you on their own. even spiders and other creepy crawlies aren't that spooky when you can just step on them. The logic of horror then dictates you have to either enhance the number of animals (like in White God, The Roost or that cockroach part of Creepshow) or make a mildly threatening animal moreso by enlarging it.
Gorillas are scary sure, but King Kong can grind you into paste. The ants in Them! have numbers AND giant size on their side. Of course, usually we are learning some lesson about toxic waste or radioactivity screwing with nature but I would argue any old excuse for a giant animal will suffice. For proof, check out the unexplained but massive spiders in King Kong and Lord of the Rings. No thank you.