Being an owner of 2 Under Armour ColdGear mocks, I was expecting much the same with this new one, however I've noticed a couple of tiny differences that improve the product massively. First, the rubber strip along the inside at the waist. It works perfectly in preventing the top riding up towards your stomach, which should keep you nice & snug. Also the inside of the top seems to be a kind of 'brushed' material, even more comfortable than the standard UA ColdGear mocks of a few years ago.
I've always worn a medium, however this one seems even tighter than usual... maybe says more about me than anything else!