The Beckton herd is the foundation herd of the Red Angus breed.  According to their sale catalog they use their bulls first on their heifers before they ever put them into their cow herd. 

Calving ease and birth weight are not the same thing.  There are a lot of people managing their herds as you say can't be done and have been doing it for long enough that if there were going to be a problem it would have shown up by now.