This piece on StratPage rather says it all. It’s kind of interesting, aircraft become more ‘efficient’ in a technical sense of cargo(including fuel/power)/weight as they grow larger…it’s mostly about geometric effects of volume vs. surface area. There is also a tendency to focus on endurance in drones for obvious reasons, which in air vehicles means slow and relatively large wing spans. Consequently the ‘low threat’ generation of UAV’s are all about the same in regards to vulnerability. But the next generation, Quadopters, UCAVs, Avenger, etc, are all higher performance, trading something away for more performance, and will be significantly less vulnerable.
And let’s be clear the first generation were all about experimentation with the basics, not about high intensity combat ops. If you look at a lot of the manned platforms in service, survival in combat has not been a big driver for the last decade.