Thats a good point. Nicer weather people tend to stay out watching them longer and end up throwing more treats.My thoughts are that with the longer days they would be eating more so the eggs should be larger, not smaller. But it's not working that way for you.
There is a correlation between how much protein they eat in a day and egg size. It's not what is in one bite but how many total grams they eat in a day. Even that is sort of averaged over a few days. Are you increasing the amount of treats they getting in good weather, especially low protein treats? I do that with wastes from my garden. You might try either cutting back on the treats or using higher protein treats to see if you see a difference.