Hola Mel!
Hard to see the actual color of your Grapefruit Fly leaves but I would speculate the smaller one is in need of nutes. You see the Grapefruit Flys are nute hogs and will take whatever you can give them-- within reason of course-- just to give you an idea-- in flowering I generally run at a high point in EC nutrient levels 1.8. Even when running nutrients this high, the Grapefruit Fly still shows some signs of nutrient deficiency! Just something to consider.
But back to your question on the Grapefruit Fly height, she will grow BIG & ROBUST; so if you start flowering with a plant 18 inches tall you can expect a finished plant 3.-4 feet tall. And she likes to "spread her wings"! But you can expect a BIG harvest of phat colas when grow correctly!
I know you do her right!
Mosca
My Grapefruit momma came from Reeferman Seeds. He used the same Sweep Pink Grapefruit clone Breeder Steve used. I have run this mother for over 4 years and she KICKS ASS!
Feliz Navidad,
Mosca