Have you heard of it? If you haven’t, I wouldn’t be surprised. I hadn’t heard of it until it was mentioned in Atomic Robo (and if you haven’t heard of that you must start reading more awesome comics).
Basically, it says giant ants can’t exist because their bodies would never be able to support them. Considering that ants are already poised to take over the world with their little, tiny bodies, I think the fact that they can’t get any bigger is a big load off our minds. Anyway, the reason the creators of Them! were thankfully wrong is because as something gets bigger, it’s volume increases exponentially. Something that’s two times bigger has eight times as much volume.
To illustrate, I’m afraid I’ll have to use math:
The volume (V) of a cube is the length of one side (L) multiplied to the third power, so L3. If L equals 4, V equals 64. If L equals 20 (5 times an L of 4) then V equals 8000 (125, or 53, times a V of 64). This works for any formula volume, whether sphere, cone or anything else.
The same applies to anything. If an ant is made bigger by, say, gamma radiation, whatever its size is increased by (2, 5, or whatever number), the volume is the cube of that (8, 125, or whatever3). But the ant has its original body structure, because what is this? Some kind of magic radiation? And that body structure cannot sustain the increase in volume. It would be crushed by its own weight.
This same principle applies to any building, animal or ship your characters will use, so keep it in mind before creating any gigantic titans battling over the ruins of New York City. Or dinosaurs.