Like twistedfeather said, yolk size is depended a lot on both genetics and nutrition, as well as other factors. I have production-laying birds (red sexlinks), which is a popular choice as a commercial brown-layer. Their yolks are often larger than storebought eggs because they've been bred to produce huge eggs. However, a Buff Orpington (which is a popular backyard choice) may have a smaller egg and yolk than storebought.
Some people believe that brown eggs are better for you than white eggs, but this is not necessarily true. Small farmers often choose brown-laying chickens, such as red sexlinks or Rhode Island reds, while the leading commercially producing chicken in the United States is the Leghorn, a white layer. An egg from a truly free-range leghorn (white layer) will be better for you than the egg from a red sexlink (brown layer) in a cage. Store-bought eggs are also usually older as well.
Raising chickens has been extremely rewarding. I put work into my birds, and it's exciting to get eggs out of that. They are also fun to watch! I also like to use their feathers for crafting, which I am well supplied with!
Good luck on your project! Really interesting topic you chose, I hope you do well!!