There are many breeds of chickens and egg size and prolific laying is based on breed for the most part. Bantam breeds produce smaller eggs, obviously, and the larger, heavy weight breeds produce larger eggs.
However…all eggs are mostly water and if a hen is not getting enough water the content of her eggs will suffer accordingly.
Just like fruit trees, the size of the fruit and it’s viability will have everything to do with how much water the tree/plant is taking in when in production mode.
Same for chickens.
The size of the yolk ( embroyo ) will be based on the size of the breed, with larger breeds producing larger yolks. Just like any other species, the size of their offspring is determined by their breed.