If you read the fine print, you can see that not all skinnies are created equal. Some calve widths are 10", some are 12", some are 14" -- you get the idea. Meanwhile, not all women are created equal, either, so what looks skin-tight on one will not on another.
Then, combine that with the fact that some like scrunch and some don't. You can end up with quite different looks depending.
I am not sure there is any absolute "rule." More a matter of preference and finding the fit that is most flattering to you, personally. I own several different kinds and like them for different reasons. Example: I like my "legging" jeans (ultra skinny) for tucking into boots, for rolling up a bit in summer to make cropped jeans, and when I want a really streamlined bottom to go with a looser top.
I like my more regular skinnies when I am wearing booties and a jacket.
I like my "slouchy" skinnies (aka boyfriend jeans) when I am hacking around on the weekend, with booties (or sandals, depending on season).
I like my straight leg jeans to fit over booties that have a gap at the top.