Info about base-layer clothing?

Ok, this may seem like a dumb question, but hey, i'm asking it anyway. I needed a pair of thermal underwer to wear under my pants, since i take the bus and walk around alot outside in the cold (it's -2 right now here in minneapolis), so i went to North Face and bought some synthetic base-layer pants. The only problem is after i took off the tags and threw away the box it came in, i realized that i had bought the wrong size. I needed an L, but bought an XL. I tried them on, and they fit, but they aren't tight against my skin. So my question is this - does it matter if your base-layer is tight against your skin? Does that affect the way it keeps you warm? I would think that having a bit of air between your legs and the fabric would actually help keep your legs WARMER than if the fabric was tight against your skin. Am I wrong? Any advice from people who wear base-layers alot will be appreciated. thanks!