So fun fact about chickens. Chickens go find somewhere to lay an egg, they will first lay down to relax as the egg literally turns inside their body to the correct direction for laying. (Wide side first.) Then they actually stand up to lay the egg.
This apparently aches for some chickens, it probably aches for some hens more than others. I've seen some people describe their hens groaning as it happens for ten minutes. Others seem to sit down for 2 minutes, plop the egg out and walk away no issue. My hen Tea Cake is in and out in five, then she sings a song about it. Most chickens seem to recover pretty easily and walk it off with no issue, though some chickens need a long sit down afterwards.
The eggs themselves are usually proportional to the hen's size. A small hen lays a small egg, a large hen will lay a large egg. Generally most home-stead breeds (fancy chickens and pet chickens etc) do not lay every day, they lay 100-200 eggs a year or so. 200 eggs is considered medium yield, but there are breeds that can reach the 300s. These breeds tend to be commercial farm breeds, and it takes a toll on their bodies. I don't want my hens (or any hen tbh) to lay at that capacity, its not good for them and can cause a lot of issues.
Laying an egg vs. a period (though they are very different don't let people call them chickens periods or I will personally be autistic at you about it) both kind of suck. But now I have your attention:
If you want to help chickens, don't buy large eggs.
High yield breeds (300~ eggs or so) laying huge eggs are susceptible to prolapse. A prolapse is when your insides become your outsides, I have witnessed a prolapse that ended the life of one of my hens and it was a nightmare. I keep hens as pets, I have time to care for them one on one and treat them around the clock. Large scale commercial farms do not have the incentive to care for individual birds when they get injured. A bird laying 300 large eggs a year is extremely susceptible to prolapse, which on a dense battery-cage or barn commercial farm (depending on their standards) could be a death sentence as they're ignored in a sea of other suffering hens.
There are a lot of fucked up parts of the egg and chicken meat industry. Not everyone can go vegan, or wants to go vegan. But we can make a difference. When I was a kid in the early 2000s, we learned about battery farming in school, and in 2012 it became illegal in the UK and later the EU. Now the UK is expanding it's welfare laws for farm animals, due to pressure from the public being more informed by organisations like Blue Tractor, Compassion In World Farming and the RSPCA. Its not perfect, we've got a long road ahead. But if we want to take steps towards better welfare, it means making informed choices.
The yolk of a large or XL egg is the same size as a small egg, only the whites become bigger. So unless you're trying to make exact measurements for macarons, or you're on the Gaston Diet, you do not necessarily want XL eggs if you're looking for breakfast.
You can learn more about this from the British Hen Welfare Trust.
Tell your friends not to buy large eggs, mention it at parties as a fun fact. Buy mixed weight eggs. Work it into conversation, inform people. Keep your own hens who lay at low or medium capacity, watch them thrive. Be really annoying and ruin tumblr posts with several paragraphs about your deep chicken knowledge. Spread the word!