Not only was the Great Heathen Army about six hundred years or so “prior” (so to speak) to “modern” Westeros, but the scales are all skewed. The Seven Kingdoms have slightly more than ten times the population of the 15thC British Isles, at least ten times the landmass, and seemingly nowhere near 10 times the number of named/notable lords. I’ve always figured that in an apples to apples comparison of land and military might between 300AC Westeros and late medieval Europe, a non-Paramount Lord of the first rank (Hightowers, Boltons, etc.) was closer to being the King of Scotland or Duke of Aquitaine than a run of the mill real world Baron.
Yes, excellent point.