Why was your stillborn so small?

I had my child at 23 weeks and 3 days gestation. the normal weight for that age is at least 1lb. & so many oz. & the normal length is at least 12 inches long. My son was 12 oz. & 6 1/2 inches long. The doctors said that he was developing normally until I lost him. So my question is why was he so tiny? He was at least .75 lbs & 5 1/2 inches smaller than other babies that far along in their development.