With the use of A.I., any sized mare can be bred to any sized stallion. It has been proven again and again that the old adage "the mare takes care of it" is absolutely true. A mare will not have a foal that is larger than she can bear because of genetics. Studies have even been done where full-blooded shire embryos were transferred into Shetland pony mares. Each of the mares foaled, without difficulty, these purebred shire foals! Colorado State University routinely uses light-breed mares for their transfer recipients of draft horse embryos, with great success.

From, "The Horse" Magazine,
January 1999
Fetal Development and Foal Growth

By Les Sellnow

<<...Back to fetal development. The size of the fetus at birth is often determined more by the mare's uterine capacity than by genetics, although genetics do kick in once the foal has been born.

In one bit of research at Colorado State University, a Shetland pony mare was inseminated with semen from a draft horse stallion. The pony mare delivered a small foal during a normal birth, but the foal soon outgrew it's mother once it was on the ground and nursing.

Two papers presented at the International Symposium on Equine
Reproduction held in South Africa in July also indicated that the mare exerts considerable influence on the size of the growing fetus. ...

...The second study was carried out by researchers in England who studied the influence of maternal size on fetal and post-natal
development in the horse. They reported that, "Maternal size
significantly affects fetal growth, presumably by means of limiting the area of uterine endometrium available for attachment of the diffuse epitheliochorial placenta."

Once the foal is born, genetics and nourishment are highly influential in growth and development. A foal which has the genetics for large size can do considerable catching up in the first few months of life, particularly if it receives proper nourishment and has been properly nourished while in the uterus. ....>>