Does the sizing take into account being...
Does the sizing take into account being able to fit them over other pants?
I'm normally a small so should I buy a medium if I want to wear there over regular pants?
No, sizing does not take into account wear them over pants. they're meant to be worn by themselves or a base layer, which you still wouldn't size up.
These are a very trim cut. Cuts down on the noise factor too.
I would definitely size up if you intend to wear them over other pants. That's what I did and this lets them be a good emergency rain layer as that afternoon storm rolls in.