"I don't understand how such a small person as yourself manages to keep warm carrying such "light" gear"

I think this varies based on several factors, to include experience, hiking "style", and overall physical condition (strength, endurance).

Thru-hiking last year I was in excellent shape by the time I hit the Sierras in early June. I had a Montbell Thermawrap jacket as well as a size larger thermawrap vest to layer, but I essentially never used the vest, that one insulation layer (upper body only) was enough. Being in good shape at the time helped in part because I could do more miles --- thus there was a lot more flexibility in where I camped each night. So I always camped below snow level.

By "style" I mean a host of things a backpacker does, but in particular here I'm talking about a process that involves more time on the trail and relatively little in camp. If camp is just a place to eat and sleep (not hang out), then much of your in-camp time is in the sleeping bag. Strategy for being caught by unusually bad weather or a situation like that: stop and set up camp, crawl in the bag.

In that context the temperature rating of your sleeping bag, whether you use a tent and what sort (single vs. a warmer double wall), what other clothing you can wear inside the bag, what insulative padding you sleep on, those all relate too.

I don't want to suggest that a person should go out with too little in the way of insulating clothing (!), but what's needed --- or at least what's desired for an adequate sense of security --- can vary a lot based on these sorts of factors. Walking the JMT part of the trail last year there was an obvious visible difference between pack sizes and overall amount and type of gear being schlepped along by folks walking the JMT only vs. PCT thru-hikers, with the latter carrying a lot less.