Basically what you wonder is how many polygons those games render at average when you walk through their levels, as detailed as in for example F.E.A.R or Half-life2??
Don't worry, F.E.A.R actually shows that it's max of onscreen polygons lies somewhere around the 500 000 polygons. It's average is somewhere near the 150 000 polygons per frame. Not extreme amounts. Source Article about F.E.A.R's engine
