I did the same thing. I bought a 4 year old Mustang mare because
mainly I liked her size (and other things about her), very fine,
petite build and 14 hands, I don't need a big horse, I'm 5'2" and weigh
105. Well, in just over 10 months not only did she fill out quite a bit,
she's at 15'2 now! And she will most likely continue to grow.....I'm going to
have to get a mounting block. (When I bought her she had been neglected and
starved and was in horrible shape, but had a fabulous attitude and
had something about her. My vet said just good feed, care and love
and she grew like a weed.)