Ripper's comment about QH cross foals made me want to post about my own observations.
I had read that foals are the only horses you can feed free-choice grain. They don't over eat on it like every other horse will.
I have raised a grand total of one foal, but I tell you what, I think nutrition is a BIG part of raising a foal to it's fullest potential.
My mare is a 15.1 Missouri Fox Trotter. She came pregnant by a QH stallion whom I've met and was very average in size and height. Probably in the 15.1 range, certainly not huge or tall.
I was very anal about feeding the mare when pregnant and then creep feeding the foal (Purina Ultium Growth) and now my "foal" is now 15.2 at 21 months of age. The string test puts him at 16.1 at maturity. He's taller than his mom, my Mustang gelding and his sire already.
I don't know where the "tall" came from but I am guessing it is because it is in his genetic make up somewhere and it is coming out due to optimal nutrition. He's stocky and muscular too, even though he was gelded at 5 months.
I know it is not good to push growth on a horse. And I haven't been trying to do that. He only gets about 1 lbs of grain a day, and ample high quality hay. Either alfalfa/grass mix or alfalfa. And a vitamin/mineral supplement. And everyone is impressed with not only his height but substance.
When I had the vet out a couple months back he asked me if I had been riding him yet and I had to remind the vet he wasn't even two yet!
I know another lady who raises registered Paints and her foals are narrow and undermuscled and not as tall as my QH/Fox Trotter cross. She doesn't have the money to feed them optimally and it shows in their lack of sized and muscle.
Here is my baby at 20 months. And keep in mind he is only half QH.