eggs - despite marketed "by size" - are sorted by weight.
if the USDA seal is used then bunches of rules apply.
each individual egg must be in a weight range
the weight of a dozen eggs must be within a range - they can't all be on the upper weight end, for example.
the usual and customary number for weight of the shell is 11% of the egg
- the white = 58%
- the yolk = 31%
which supposedly varies only slightly by peewee to jumbo....
details, lots of them, here:
https://www.ams.usda.gov/publications/content/egg-grading-manual