Presumably they are like the windtops worn by cyclists?
I find windchill much more of a problem than lack of insulation. A decent windtop to keep the wind off your torso is a big help.
I have a very lightweight windproof gilet (sleeveless jacket) which packs down smaller than a medium-sized apple. It makes a big difference when there is a cold wind blowing but doesn't make me overheat when out of the wind or exercising hard.