egg size

hi all my hens are laying lovely now around 19/20 week old feeding them layers pellets plenty of fresh water and a treat every other day of mixed corn , mealworms or veg im getting lovely golden yolk eggs but there still small what sort of age will i be getting bigger normal sized eggs

My girls are just a bit older than yours (8-10 weeks) and they are laying quite big eggs now (well 5 of the 6are) and one has laid an enormous double yolker three weeks running!