We got this one from one of our girls today. The one on the right is a normal sized large egg, and it is a standard ole lime from the grocery store above for reference.
I get xlarge eggs from my 2 red sexlinks almost everyday. I have an egg scale and they are either x large or xxlarge. Then my RIR's usually lay large. My banties lay Pee Wee.
From the coloration, I think it most likely came from one of our buff orpingtons. We also have some red sexlinks and barred rocks in there. The BR tend to lay smaller ones, and the RSL (who usually lay the bigger ones) seem to have a darker egg.