Lack of sufficient fresh water supply may also cause smaller eggs.
My French Marans usually start with 54 grams, sometimes even 59. After a few weeks they reach the mid 60 and the following year most eggs have 65-75 grams.
I was wondering the same thing but today when I collected my 3 daily eggs two were big and one small, Guess it’s a waiting game. I have 6 only get 3 daily, Not sure if they are from the same girls each day. what is the suggestion for % of protein my bag says 16%.