so you measure everything in cups before you put it on the plate.. and how big is an official egg?
well never mind, have fun with your measuring system
Maybe it's different in other countries, but in the US there are standard egg sizes (M, L, Jumbo, etc).
Regardless of that, the nutrition facts are printed on the egg carton itself
for the eggs in the carton. It doesn't matter if the size varies from Whole Foods to Kroger to Wal-Mart eggs because the serving size is specific to that item from that store. It can be tailored to whatever is most appropriate for the way the item is bundled, instead of using 100 g, which for many things doesn't make sense as a unit of measurement at all.
So no we don't measure everything in cups, but the things we do measure in cups (cereal, pasta, etc) will have "per cup" nutrition facts. Cookies or crackers from a box will have "per cookie" and "per cracker". Loaves of sliced bread will be "per slice".