difference in the breed depends on the EPD you would look for but if your just looking at actual birth weight I prefer calves less than 80 lbs because if you get much bigger look to be having fun pulling calves
There is also a great deal of difference within breeds. According to the 2002 American Gelbvieh Assn. Sire Summary, there is a 22 lb. difference between the highest and lowest Birth Weight EPDs in their data base.
It also depends a great deal on enviornment.
Studies have shown that calves born in winter will be heavier than summer born calves.
Research has also shown that cows fed high protein diets during the last trimester of gestation have significantly higher birthweights. We live relatively close to a soybean processing plant and can typically buy soy hulls at a discount to hay, so we feed six pounds per head per day of soy hulls during winter months to the cows. Since we started feeding soy hulls are birthweights have gone up 6 to 10 pounds. I attribute it to the highly digestible protein in the hulls. Others who also feed soy hulls have seen similar results.