Yep, perfectly normal. All babies are different, just like adults are different. We all have different needs and appetites. As he's much smaller, you'd expect his appetite to be smaller as well. My daughter was 6lbs 9 oz at birth and when we switched to formula at 10 weeks, I don't think she was having 16 oz a day even then. So just keep doing what you're doing and it sounds perfectly right to me.