Still getting small eggs

I have 8 chickens, 2 Barred Plymouth rocks, 2 White Plymouth rocks, 2 RIR's, 1 New Hampshire red and 1 Black australorpe. They have been laying since October of 2013. I get about 6 to 7 eggs daily, give or take an egg. About half of the eggs are large to extra large and the other half are small to medium size. I keep the smaller eggs for ourselves and sell the large eggs. They get fed Purina Layers pellets, BOSS, red wheat seed and oat seed. They also get all kinds of stuff from the garden such as: corn, squash, swiss chard, beans, tomatoes, peas, raspberries, grapes, cherries, etc. They also get worms, meal worms, grasshoppers on occasion and grasses. They all seem very healthy, just wondering why I would not be getting all large to xl eggs by now. Wondering if it is something I am doing wrong or if that is just the way it is. Any help would be appreciated. Thanks