Usually, there are other variables...amount of protein ingested rarely leads to smaller eggs, usually only thin brittle shells or shell-less eggs. The yolk and albumen are produced first with the shell forming later around them.
Smaller eggs are usually caused by breed, age, stress, heat, or other outside sources.  I've used Rural King for 3 years and my Bard Rock and Ostrolorp eggs are so large that I can't completely shut the Jumbo egg carton lids. Shells are a bit weaker than store eggs, but strong enough that I rarely get accidental breakage. Nutriena is a good food-but at exactly double the cost of Rural King, I'll take my chances...