You have to remember this breed was created to replace small turkeys in the roaster.  In other words for meat, so breeding emphasis wasn't placed on egg production.  
line breeding creates smaller egg producers.  If your breed function was duel purpose or egg production , it would be proper to cull for egg production as well as conformity to the physical characteristics (though hardly anyone does in the US).   However, in a bird that main function is to be a meat producer, then the physical characteristics become the paramount culling criteria. 
Both the NJG and the Marans I have (all under 1 year old) , do not lay anywhere near the size and with the consistency of my friends ISA browns, of the same age.   
If I keep the Marans, this is something I will have to try and work on, their breed function is more of an egg producer than a meat bird, and so I am disappointed in their egg size.