If a young foal looks like a tiny version of an adult horse, it's probably going to be a small horse when it's full-grown. If a young foal reaches the age of two without having gone through an awkward, gangly stage, it's probably going to be a small horse when it's full-grown. The gangly and awkward ones that first seem to have huge heads and huge joints, then grow up one end at a time, first croup-high, then high in front, then croup-high again, and that go through a stage during which they look long and bony and uncoordinated -- those are the ones that will probably be tall when they are full-grown. This would tie in with your source that mentioned a small head indicating early maturity -- it doesn't mean that the horse will actually be physically mature earlier, you'll still need to wait for the skeleton to finish developing. But I would interpret that statement as meaning that a foal that develops adult proportions very early is a foal that is going to grow up to be a small horse.
Questions... Do any of you feel this has merit?
Have any of you heard of this?
How much is probably genetics?
What have been your views on how horses grow in your experience?
My Toby who is 2 years old now. His dam is 15.1hh. She is our palomino. His sire was 16.1hh. Toby is currently a little taller than his momma. Toby grew like it was mentioned above ... gangly, goofy, one end before the other. I did not do the measurement of his leg thingy that a gal told me about to get an approximate height because it was past the right time to do that when she told me about it... follow me? So, how long does a horse continue it's growth (up not out)?