This isn't necessarily true. Whether or not a mare has trouble foaling has little to do with her size.....the important factor is whether she is roomy enough "inside" (wide hips, etc) to foal easily. You can have a vet check this out very easily. There are many tall mares who just aren't built for foaling, who would have trouble no matter what size stallion you bred them to.
This being said......as Rabbitfizz already mentioned....the ONLY problem with breeding the under 30" mares, is that if there IS a dystocia (which can happen with ANY sized mare) there is very little room in which to manuver the foal.