It's tough to get both size and egg production. There is a documented negative correlation between growth rate (rate of growth, final body weight, frame size) and reproduction. This includes egg production as well as fertility and hatchability. Not saying you can't work towards both, but it takes effort because size and reproduction work against each other. If you concentrate too much on size and less on reproduction then reproductive characteristics will diminish. We often see this less in fertilty and hatch because we cherish any egg laid and if the offspring meets the SOP we keep it as a breeder and often ignore its mothers rate of production.
Can you get both? Sure, but exhibition poultry enthusiasts want birds that meet the SOP much more than they want a bird that lays lots of eggs. So there goes the egg numbers with each successive generation.
I think genetics are a part of it , but I think too that how these big birds are raised has a lot to do with it . I had a lot of breeders question me when I said that my English Pullet was laying at 6 mos, as were my SOP birds. Well, I have chicks growing out to prove it. The girls are consistently laying an egg a day. I'm not about to ask for more !
You cannot raise these big birds properly in little pens , or my hated "Chicken Tractors ". From at most 4 weeks , depending on weather , my chicks are out running around a good half acre of mixed grasses . They FLY as babies , and turn into little hoodlums , who bother the biggies . All the while they are developing frame , and muscle to hold that frame together . Mine eat me out of house and home , until they start to lay , when they settle down . Eye of Newt is expensive !
Hatching rate has been very good , once my cockerel figured things out , at 6 months old . I started plucking , but stopped that , as I figured new owners would not want to have to pluck , and I might want to show a few . After a couple of weeks , fertility returned to normal .
My feeling is that I am raising CHICKENS , not KOBE beef . Big chickens , whom I let be chickens .