My White Leghorn lays the biggest egg I get and she is my smallest hen.The large dark brown eggs are RIR's and the smaller lighter eggs are the Barred Rock's. I do think their (BR) eggs taste the best.
I have one RIR hen that lays an egg each day that you would swear is a double yoker but it isn't
I also havea RIR that lays a huge egg everyday. Too big for the large egg cartons. All the others fit perfect in a large egg carton. The eggs did start out on the small side when they were pullets, but now that they are full fledged hens the eggs are consistently large. I have 6 BR and 5 RIR by the way. Oh and four 14 week mixed breed (hopefully all pullets and colored egg layers)
Henderson's Chicken Chart has some stats on laying and egg size.
According to their tests Delawares laid the largest egg. I have had many different breeds and my biggest eggs came from an old Austrlorp hen. My black giants laid a medium egg. Nor even worth considering for size. I knew a guy that raised Turkens and I was amazed at the consistent size of their eggs.
Of my current flock, the Welsummers not only lay the biggest eggs but also the most.