If you are talking white eggs, they are from Leghorns, commercial brown eggs are from red sex-links. No flock is perfectly uniform. When a flock is laying a majority of large eggs there are always a percentage that are x-large and jumbo, some even larger than the standard jumbo. The average egg size goes up as the flock ages. The ones that are much larger than than jumbo are usually pulled and not processed as they don't fit through the processing equipment too well. The eggs are sorted by size and grade. Most large, Grade A eggs are sold as shell eggs, with most small, medium, x-large, jumbo, and Grade B eggs being sent to the breaker to be turned into other egg products, although some of them are sold as shell eggs as the market permits, but the large eggs are pretty much the standard and the most popular.
Somebody mentioned that those eggs are sometimes thin-shelled and porous which is a sign of poor health. It's not necessarily due to poor health, many times they are thin shelled because the hen has laid an egg much larger than her usual egg and just doesn't have the resources to wrap it up in a nice shell. As the eggs get larger, the shells generally get thinner. That's just the way it works...