A few years ago there was a study out of Colorado State that said the largest contrubitor to high birth weights was the protein level fed in the second trimester. If you think about the timing of the 'fall flush' that a lot of areas get in the fall or when winter or fall feeding starts, it makes sense. As usual I can't find the paper now.
10 years ago I walked into a vet clinic as they were starting a c-section. Not being in a rush and not really thinking, I stayed to watch. When the first foot came through the side, the vet hollered for help. It took three of us to get that 196# Maine-Anjou out of that cow. Thank goodness I wasn't wearing good clothes, it was a mess. Both lived and thrived that year, though the calf didn't set any records.