Not to say that they certainly can't grow taller, but my colts this year are all out of 38" mares (probably a smidge taller now, but they were all 38" at time of permanent measuring) by a 35.75" stallion; Junior measured 22-23" tall at birth (June 17) and Monte measured almost 24" (June 23), didn't get a birth measurement on Manny (july 12), but he was smaller than the other two and still is. One September 28, I weighed and measured all three boys (you can do the math on their age): Junior was 29.5" tall (about 110#), Monte was 29.5" tall (about 105#) and Manny was 27" tall (about 75#).
And, some grow fast early and taper off, other grow slowly over time.