You aren’t just heating up your own body w/ homeostasis, you’re heating up the air around you as it radiates off of your body. You will be warmer with tighter fitting clothes. Looser clothes can help with air flow, but if the air is -10°F air flow isn’t your friend.

Insulation is about trapping the air your body has warmed up next to your body, so you don’t need to constantly spend energy heating up cold air.

I’m not sure what sort of activity you’re planning on so I can’t give very good recommendations on exactly what to wear. But I would say just buy clothes that fit you. You probably shouldn’t be wearing so many thick layers that it requires you to go a size up.

Also keep in mind if you’re so warm you start to sweat, once you cool down that sweat is going to make you feel even colder.

Again wear wool, Merino wool if you can. Don’t wear cotton.

Emphasizing what the other poster said, you don’t want to compress anything that traps air. Your best insulator, by far, is trapped air.

Proper mid to outer level layers are made to size so they fit on top of the lower levels

The only tight layer is the first.

My winter set up is:

First layer: tight top + bottom wool long johns (in sure there is a proper term for em, but it’s what we called em)

Second layer: loose wool onesie I got from Roots.

Third layer: loose sweat pants + hoodless sweater

Fourth layer: loose insulated work pants + work hoodie

Fifth layer: snow pants and jacket

This gets me through winter in Winnipeg and rarely do I get cold.