600g for a 1 1/2 year old is not bad at all. That is a good weight gain. Many people have a goal to get their girls to 1500 or more grams by the time they are three. Many reach 100g by the time they are two. (I am going off the average based on my snake collection. It may be totally different for others). My one year old (turned 1 year old May 4) is currently 815g which is a bit much but nothing to be too concerned about. They grow pretty quickly in the first 3 years of their life and they still have a noticeable growth up until they are 5.