2 year old is not gaining weight?

My daughter did not gain weight well either. She is a tiny, little thing, God lover her. Anyway the Dr. told me to give her some nutritional drinks (like pedia sure). I bought the pedia sure and it was way expensive. I compared labels with pedia sure and ensure/boost and both are the same except ensure and boost have twice the amounts of everything pedia sure has. So I just gave her half of a boost or ensure a day (whichever was on sale). Now she is gaining weight much better and there is no need for the nutritional supplement.