First, you could lose most of that grain. Chicken feed already has corn/grain products as the primary ingredients. It is complete nutritionally and adding more grain won't help.
When reading the following, keep in mind that laying pellets are a complete feed for birds actively laying eggs.
16% protein, approx. 4% calcium and the entire complex of vitamins, minerals and fats contained therein are what laying hens are known to need and in the proper ratios. The same goes for other feeds that are formulated to provide optimal nutrition for the age and species they're intended to feed.
After over a century of exhaustive research, it is well known what nutrients chickens need at various stages of life and production. All feed manufacturers have this information and is why each bag of feed has a statement to the effect, "This is a complete feed and should be fed as the sole ration, no supplements are necessary."
Mixing layer feed 50:50 with grain results in a mixture that is about 12 or 13% protein and 2+% calcium.
It also diminishes the optimal vitamins and minerals that were in the primary feed.
That level of protein is not sufficient for body maintenance and good ovulation. That's likely why the egg size has decreased since the albumen and yolk contain lots of protein. Egg size starts with a follicle which grows into a yolk and then albumen applied dependent on available amino acids in the bloodstream.
The calcium carbonate applied in the shell gland goes around what mass is there already. The egg can't grow larger at that point because the first stages (yolk and albumen) are already complete.
If they are able to continue laying, the calcium you are providing could be insufficient for shell quality. That's probably why they are scarfing up the crushed egg shells.
If egg production slows, the calcium may be sufficient to provide good shell quality.
Usually when vitamins are in short supply, the first deficiencies will appear in the water soluble vitamins (primarily B complex). Luckily in your case, the wheat is supplying much of that but may still be insufficient for the growing birds' needs.
The fat soluble vitamins stay in the system longer, so deficiencies take longer to manifest themselves. Those would be vitamins A, D, E and K.
Since you have roosters, perhaps you want to hatch eggs to replenish your flock. The diet you are currently providing is not adequate for good hatchability. The nutrition in a hen's bloodstream is what she has available to deposit during ovulation in the yolk and subsequently in the albumen. What nutrition is in the egg is all an embryo has available to grow properly, reach full term and to be a viable vigorous chick.
What I haven't addressed yet is the roosters and chicks.
The roosters are probably fine with the protein they're getting.
Your young birds aren't getting the nutrition they need. IMHO, 2 month old birds should be getting about 18% protein and the vitamins contained in a complete feed. You don't say how long you've had these birds or how long you've been feeding them this way. Assuming that the chicks are being fed what the layers are fed and assuming you have been feeding the chicks this way for a month, they may now have stunted growth. If you've been feeding them this way since day old, they definitely have nutritional deficiencies as well as having been getting too much calcium.
I'm not a proponent of or an apologist for the feed industry. But the simple truth is, the simplest and cheapest way to feed chickens what they need is by giving chicks a starter/grower feed and laying birds a complete layer feed. If feeding layers, roosters and chicks in a mixed flock, feed an All Flock or grower feed to all and provide a calcium source in a separate container so the hens can supplement their needs.
Mixing things with feed is an unnecessary added burden and diminishes optimal nutrition.
Attempting to make one's own feed at home isn't cost effective or better unless one has access to lower priced ingredients than most people have AND they have a means to analyze the nutrients in the final result - like the feed companies do.
Scratch grains should be just that. Grains scattered about for the chickens to scratch for but still no more than 5-10% of the diet.
You are fortunate to live in a climate that has decent forage this time of year. Most climates in the northern hemisphere have little of nutritional value available now.
