Yes, as it happens.
Draft stallions tend to produce more semen...but the same amount of sperm, meaning their sperm count is lower. Then that sperm has to get through a bigger uterus. Draft mares are also more prone to endometritis (uterine infection) and retained placenta. (This is also why draft mules are more expensive - it can take quite a few coverings to get one).
Most draft people think it's worth it, though.