It's because hatcheries breed for one thing - Egg production.
Show Quality breeds go for the standard, which includes proper weight, size, roundness, height, etc. They go for the original purpose, quite often being dual purpose, which requires some meat on the bone. Hatcheries don't really care for this. They care about having chickens that lay enough eggs to sell enough chicks, that's about it.
I too was very shocked when I met my first non-hatchery birds. I thought there was something wrong with my flock! But, not really, it's just poor hatchery breeding.
Consider this - My Brahmas from Murray McMurray weigh 5-6 lbs. A normal Brahma hen should weigh 9 lbs. True Ameraucanas should be a moderate sized breed, hens around 5-6 lbs. . .
Check this out. Isn't it just sad?
The hen on the far left is a Brahma, who is being dwarfed by a blue Ameraucana just to the right of her. The silver laced birds are Wyandottes, who also should be bigger and FAR rounded and fluffier than Ameraucanas. But instead, they're gamey and small. . .
This is a Brahma at the same size as an Araucana, a smaller breed than even Ameraucanas.