My time-distant farm chicken background tells me that the feed the chickens were laying on determined the color and size and taste of the yolk. Egg yolk is about my favorite food in the world. I don't remember egg size being a hen age thing, but we didn't let ours get too old anyway. The eggs were all different sizes per hen throughout the laying seasons.
Edit: these were yard ranging birds, not cooped or caged. Bugs and yard scratch were large part of food also.