Reasons for Reduces egg size in Laying Chicken

Reasons for Reduces egg size in Laying Chicken
There are quite a few things that affect a hen’s ability to lay eggs such as her age, genetics, nutrition, daylight hours (daylight hours, however, will not make a difference to the size of the egg) and living conditions. Small eggs can often be a sign of stress due either to an illness or something in the hen’s environment, such as temperature extremes, constant threat from predators, moulting, attacks from other chickens or restricted access to fresh food or water.
✍️ Genetics
This is clearly a cause of small eggs in some circumstances, such as bantams.
Otherwise, our hybrid hens will normally lay a medium/large egg when into full lay.
If the feed is not rich in nutrient,  the egg produced would be drastically small. 
Because the laying chicken have required nutrient which must be provided. 
Hens should also have free access to oyster grit to give them lovely strong eggs and access to fresh water is very important to egg production as the egg is mostly made up of water.
Rarely, small eggs can result from a nutritional deficiency, such as a lack of salt.
Our hens are highly vaccinated to commercial grade so it is very unlikely that they could contract an illness causing them to lay small eggs. Egg laying can be affected by the stress of having worms, lice or mites. Treat every two months for lice and mites and worm your hens regularly.
Hens are happiest when free ranging, but if not, provide them with distractions such as extra perches and vegetables hanging up outside to peck at. Make sure you clean them out regularly.
Our hens will continue to lay throughout the year and they may lay a little less if it’s really hot.  This is to show that heat affects egg production 
A moulting hen will slow down laying or even stop while they grow new feathers depending on the severity of the moult.
The other factor is attacks from the rest of the flock although. In any event, try to make it a habit to observe your chickens frequently, watching them interact with one another, and taking note of their living conditions. When you observe your chickens regularly, you are more likely to notice a stressor, and will be able to tend to the problem sooner.