Don't assume that her lack of nutrition will automatically mean she will be smaller. We made that error with our boy, and had been told he "should have matured to" but due to his circumstances as a colt probably won't reach 17hh (told to us by the person we bought him from)
The breeder of my Clyde mare looked at him as a 4 yr old and told us not to "write this one off as short... He will be ample sized, but may be late.", sure enough, He shot up about 3" between 7 and 8 years old, and is now filling out. I have to get myself a proper stick, but he definitely reached 17hh, possibly a bit taller.
My mare never missed a meal in her life and has grown fairly consistently the whole time, she is finally appearing done at 7... But her shape has started to change