Higher quality materials and filling
The thing with insulation is that it’s just a way of trapping warm air. With down you get lighter weight, smaller pack size and better durability. With better quality synthetics, some of that plus better resistance to water. But for ‘standing around in a field’, you just need fabric that keeps the wind out and the filling in and enough volume to trap sufficient warmed air to keep you comfortable.
Insulation’s basically very simple: it’s a layer of trapped air warmed by your body that stops more warmth escaping. If the shell fabric isn’t windproof, wind and air movement strips that pre-warmed layer of air away and your body has to re-heat it.
It’s the reason that when you first get in a sleeping bag, it feels cold. Your using body heat to warm the air trapped in the insulation. Once you’ve done that, you stop losing heat and feel warmer. It’s also the reason that thin insulated jackets aren’t as warm as thicker ones all else being equal.
Construction makes a difference too. Seams and unbaffled zippers lose heat unless the construction is box wall, so there’s no thinning at the seams…