Hi Chookoz - welcome to the forum
It is quite normal, as they develop the egg sizes will become more consistent and larger. They invariably reach egg size maturity at about 30 - 35 weeks but this will depend on breed and feeding regimes.
Also be aware of some birds being inconsistent with egg size, my goldline lays doubles some days and normal size on others, also be prepared for soft shell egs in the first lay or two. It is all about their bodies maturing and adapting.
If you Google "Poultry egg sze" there is a lot of research on egg sizing and feeding regimes produced by the poultry industry.
There are many experienced chook keepers who can add to this