Weight is related to food, so if the animal is not fed aproprietly, a poor feeder,fasting or not fed frequently enough it will be smaller than another animal. Genetics plays a part as well.

My advice try not to compare weight it's about proper feeding and good overall body shape, weight is a number.

And there is no age/size chart those are made by inexperience people for inexperience people, weight can vary grestly from single to double.