My MIL had a malnourished filly that actually kept growing a couple inches beyond what she expected. She was hunter pony sized, barely 14-14.1hh, but she stayed butt high for a long time and slowly grew up before she started filling out width wise.
I used to ride an Appaloosa whose parents were in the 15hh range. He topped out at 16.2hh with a butt so big and wide you'd think he was a conditioned racehorse.
You can never really truly tell what they will top out at, just guess-timate when they will stop growing upwards (when they stop being butt high, maybe).