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.
Our pullet eggs tend to weigh 38-40g and are classed as "small" according to the British Egg Information Service. Our laying flock tends to lay eggs which range from 54g (medium) to 73g (very large), although we do sometimes get the odd huge or tiny ones. Our laying flock ranges from 2.5 years of age to 9 months of age. The larger eggs will generally be from the older hens.
We wouldn't normally expect hybrid hens to start laying well until 23 weeks. Hens which start laying earlier (usually due to having had good weather and due to the time of year) will lay smaller eggs for a bit longer than hens which come into lay in the winter months at 28-30 weeks of age. No genetic fiddling has yet managed to outwit Mother Nature!
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.
Layers pellets can be fed from 16 weeks. If you want to use growers pellets to begin with, you should only use them up to 18 weeks of age. Layers pellets, provided freely, should be fed to the hens as soon as they start laying as it delivers the correct level of nutrients (in particular protein and calcium levels) to enable them to lay good eggs.
If your hens are free ranging, presumably with plenty of room to roam so will scratch out a good deal of their nutritional requirements, but layers pellets will keep them in top production. They should have access to grass.
Try not to feed your laying hens very many treats (such as corn) or they will have too much fatty tissue in their abdomen. This will cut down significantly on the number and size of eggs they are able to produce, plus it isn’t healthy for them. Corn and sunflower seeds are good for providing extra calories during cold weather, but it's best not to feed any more than 2-3 small handfuls a day for 5 birds during the summer.
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 (every month in the summer) 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 winter (maybe not quite so many eggs) and they may lay a little less if it's really hot in the summer. A moulting hen will slow down laying or even stop while they grow new feathers depending on the severity of the moult.
The other factors could be constant threat from predators which could be as seemingly insignificant as being watched by the neighbour's dog, RAF fighter planes going over regularly, buzzards living nearby or living near a busy road.
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.