... basically, how do they even work? If I wanted to make the foot collision area wider (preferably to spread out the weight of an actor more than the foot defined in base.rte) how would I change it? Would I just widen it horizontally, or are things weirder than that?

EDIT: Well okay, this wasn't too hard to experiment with. I got it figured out.

To explain the problem and solution for posterity: I made an AHuman actor really heavy, almost as heavy as a dreadnought (which weigh in at almost 500 kg. My actor was more like almost 400). Problem was, this was a bit heavy for the default limb path push forces you usually see on most actors, which I was using on that actor. So, I doubled the push forces in the hopes it'd help the actor stay upright and climb around easily.

This caused the actor to just SINK into the ground, tearing through it like butter. Obvious this was no good. So in the hopes that a wider standing surface area would alleviate this, I made my own foot and hand groups that were six pixels wider horizontally (as if you added three pixels in each direction onto the default foot and hand groups defined in Base.rte in Actors.ini).

Thankfully this worked. Now my actor doesn't sink into the ground and can climb around just fine. So I guess that's the lesson here: if you make something heavy and want to increase its push forces to cope with it, you have to widen the foot and hand atom groups too. Maybe some people already know this, but I know that on more than one occasion people have been confused about issues surrounding hand and foot groups, so I thought it might be handy to explain it concisely.

Perhaps this info could go in the modding tutorial sticky.

In fact, it's usually good practice to increase them vertically as well, specially with very big actors. Otherwise, there's a chance the feet will clip through terrain.