What are the concerns in breeding small mares to larger stallions?

 Basically, none! 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. ...