Q: Why are grocery store eggs so flat compared to eggs from my hens?
A: Eggs from your own hens normally have very "tall" yolks because they can be so very fresh!
It's not because they are pasture raised, though. The nutritional content for your own eggs when your hens have access to pasture is much better than regular "grocery store eggs." You can visually observe the difference between eggs from hens raised on pasture and eggs from factory farmed birds, because pasture raised birds will have yolks that are dark and orange, as opposed to the sickly pale yellow of grocery store eggs. Notice that in the photo above? That's the natural color of egg yolks when the hens who laid the eggs are raised on pasture.
However, pasture raising isn't the cause of a flat yolk. Even your own eggs can go flat if they get old. Eggs from the grocery store that have come from factory farms will have thin, flat yolks chiefly because they are normally pretty old by the time you get them.
Over time, the membrane around the yolk--called the vitelline membrane--absorbs water from the albumen (white) of the egg. That causes it to get weak and stretch out the more time passes and the more moisture it absorbs. As it stretches out and grows weak, the egg yolk will be flat when you crack the egg, and it will be more likely to break when you crack the egg, too.