That may be true, but the SW Okahoma elk just do not grow as large as Rocky Mountain elk. In 20 years of hunting down there I have never seen one dressed over 550 pounds and that was rare. I have seen them in Colorado where they only get the high meadows grass above say 8,000 feet and they dress well over 700 pounds. Just don't think they have the genetics SW Oklahoma. I have seen huge racks come off the refuge and the deer may only weigh 135 pounds. I also hunt far NW Oklahoma and those same racks would be on 200 pound deer. And Ft Sill actually does plant all types of farm crops, in fact they have farmers who actually lease the land and plant crops, so the elk do get supplements. If you drive down I 44, just past Hwy 49, on the north edge of Fort Sill you can often see elk grazing there. I think it is planted in alfalfa now, so it may be genetics as much as food source. FWIW
IMO it's probably some of both. If you look at most species the farther north and the colder the climate you get, the heavier the animal. Look at deer in Mexico, they're tiny compared to the northern U.S. I saw a 220lb pound doe checked in Indiana, that was field dressed. It was considered a nice doe, no doubt, but not breathtaking to the locals. I saw a fawn from that spring that would probably go 90lbs live, it looked like a little cow that one! Granted we were hunting farmed land, very predominately corn, but down here or in Texas you just don't see that happen.