Green iguana help???????????????

so i own a 2 year old green iguana. Idk what it is but i call her a girl ha. Anyway I read she should be about 3 feet long, my iguana is about half that! i feed her collard greens, kale, mustard greens, bell peppers, and sometimes i offer her zoo med all natural pellet food ( which has collards and other greens in it ). She has a uvb bulb, zoo med 5.0, and in the summer she goes outside alot. i mist the cage every day and when i feed her i spray the veggies so i know she gets enough water. RIght now im builidng her a a cage 4 ft wide and 5 feet tall. But until that is done she is staying in a 20 gallon tank. Why isnt she growing like she should be????? i do admit there has been days where i forgot to feed her cuz i got caught up in school and stuff but i dont think that would affect her that much. What could i do to get to her grow faster???