Perhaps change of seasons/ length of days.

Day length and weather have certainly changed! I think you'll also see a decline in growth if you don't keep the plants picked. I've noticed that if I leave all/most of the peppers on the plant when they're ripened, then the plant seems to stop producing new peppers and those it does produce are smaller. My other scattered thought for what could be causing it is thrips.... but then you should see other signs like deformed new growth and scars on the fruit.

Day length is an interesting idea. But overproduction doesn't seem right. Months ago, new large peppers happily formed when there were many other peppers on the plant. No scars on the fruit, and noone flying around, though some peppers are a bit deformed.

What's the NPK on your fertilizer? Excess nitrogen might result in smaller (and more numerous) fruits, in some cases. I read a study about that once.

The plants might have needed pruning at some earlier point. The fringes of older, tired plants sometimes develop smaller fruit (but if pruned at the right times and in the right amounts, I think that could avert that problem, by helping them maintain vigor).

Harvesting more often might also help. If you let every pepper ripen red, that can reduce vigor, IMO, and reduced vigor can reduce fruit size, IMO. You're certainly free to disagree! :)

Any disease or pest symptoms? Nematodes?

Is it windy? No wind at all? I wonder if a lack of wind might decrease the pollination rates, resulting in smaller fruits.

How's the humidity level? Any different?

Any anomalies?

Never heard that pruning might be needed. That's interesting. Actually, this episode coincided with some of our worst heat, and the plants did wilt between waterings. Unusual for peppers, but this was blistering heat. So I'm figuring it was probably stress. We'll see where they go from here now that the weather has cooled a bit.

Decreased pollination rates make smaller fruit? That doesn't make sense. Either you get a fruit or you don't.

@daninthedirt With regard to pollination, each unit of pollen corresponds to a single seed. I would think the amount of pollen that successfully fertilizes would make a difference. You could be right, though.

Yeah, I always wondered why some kids come out smaller than others. Must have been the wind.

Low potassium might be an issue, if you hadn't been fertilizing, anyhow. Potassium is less available during the heat, and it's said to help fruit size.