From what you describe I don't see anything that you are doing / providing for your iguana that seems harmful. I would not put too much stress into the growth for now. If you continue to provide fresh / moist food based on the food guide on this site, right temp and UVB source you should be fine. I am a first time iguana owner myself, I've followed the advice from this site and my iguana is almost 15 years old. She does eat less / less active during the winter. This year she spent most of the summer / fall outdoors. Before you know it, he / she will outgrow that terrarium.