Generally speaking, like others have said, the mare will only carry as big as she can handle. I bred my 15 hh mare to a stallion who was 16.1 hh and she foaled with no problems. That foal is just over 3 mos old and is 12.2 hh at the withers, 12.3 at the hindquarters and has string tested to be 15.3 at full-grown height. My mare's dam was 14.2 hh and her sire 14.1 and she matured to 15 hh.
I think it's a combination of genes and nutrition and even so, you can breed 2 tall horses, feed the resulting foal well and still wind up with something barely over 14 hh.
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