Here in SE Wyoming we are 100% rangeland grazing on native range with a couple of areas of domestic grass seedings. I figure it takes 2.5-3.0 acres per cow per month (animal unit month or AUM) depending on the pasture. We have about 12 pastures and try to stock them so that the pastures last 1-1½ months with about 3" of stubble remaining then move to the next. This seems to provide enough cover for the range and cows have to spend too much time hunting for feed if we graze it any shorter. By managing pastures this way, I have cows from 1100 lbs. to 1500+ lbs. that will maintain their weight and the calves wean off pretty close to 50% of the cow weight. Any cow that raises a dink is on the cull list. I am sure some of the smaller end cows can eat as much as some of the larger end cows, but there really is no way to measure it. I am betting that in 5 more years the average weight is probably going to converge in the 1300-1400 lb. cow average. Since we calve in March, we do feed about a half ration of alfalfa until May when the grass comes on. No cow gets special treatment -- they all run together.
Raising Angus seedstock, the bull buyers in this area can talk moderate cow size all they want, but when they buy bulls, they won't buy the small frame end of the bulls because they want the weaning weights. The 1300-1400 lb. cows tend to balance the productivity and efficiency on average under range conditions while producing marketable bulls. Marketable bulls is where the profit lies. Their sisters become replacements, so we have to watch our mature size and resist keeping the heifers with big weaning weights unless we are sure they will have a reasonable mature size. Everything else basically meets their expenses unless we add value by feeding out steers and heifers for freezer beef. Feeding puds isn't very profitable, so the cow base has to have some performance no matter their size.