Something that hadn't occurred to me - it seems that breeding chickens to produce large eggs can cause problems. See Times article. A couple of excerpts below.
Phil Brooke, of Compassion in World Farming, said: “Selectively breeding hens for high productivity, whether larger eggs or larger numbers of eggs, can cause a range of problems such as osteoporosis, bone breakage and prolapse. We need to breed and feed hens so that they can produce eggs without risk to their health or welfare.”
Christine Nicol, Professor of Animal Welfare at the University of Bristol, said: “There is no strong published evidence of pain in egg-laying hens but it's not unreasonable to think there may be a mismatch in the size of birds and the eggs they produce. We do often spot bloodstains on large eggs. As a personal decision I would never buy jumbo eggs.”