I don't want to highjack the other thread so thought I would start a new one. I have been breeding American Shetlands and American Miniatures for 9 years. I have unfortunately seen first hand what happens when a much bigger stallion is bred to a smaller mare. Its not pretty and usually you lose both the mare and the foal. If you are lucky you will only lose the foal.
Over the years I keep hearing over and over of this study done were shetland mares were bred to draft stallions and the foals were fine. I have never been able to locate this study. If someone has it would you please post it??? Im beginning to think its an urban myth.
The bad thing is so many people believe this and use it as just reason to breed a big stallion to a small mare. You can get lucky and have it work, or you might not.
Heres an excerpt from thehorse.com on this issue
In regards to the mare size vs stallion size problem, Carla L. Carleton, DVM, MS, Dipl. ACT, an associate professor in equine theriogenology at Michigan State University, says that although a mare has the internal ability to modulate the size of the fetus that's developing in her to some degree, "When you get a discrepancy and the stallion is quite a bit bigger than the mare, the mare's internal control over fetal size may be insufficient to the task. If the fetus' size exceeds that which may easily pass through the mare's birth canal, it will present as a dystocia due to fetal/maternal disproportion.
And NO this is not just an issue in miniatures and ponies. My good friend down the corner runs a state of the art foaling facility. This spring I went to chat and she told me how she lost her favorite mare this year due to the foal being way to big for the mare to foal out. They did cut the foal up to get it out but the mare died anyway. This was a standabred to standabred cross. The stallion was only a few inches taller then the mare. But unfortunately this was the stallions first foal crop and they found that for whatever reason this stallion produces huge foals. He was gelded.
No way will I risk the life of my mare to breed a big stallion to a small mare. Just not worth the risk.