It could be several things. Some varieties of peppers are just smaller than others, but it sounds like you have grown them before.

How big are your pots? That can be a real limiting factor. I try to give mine at least a 15" diameter pot.

Then, it could be nutrition. Are you fertilizing regularly, or has it been more than a month? I give mine Osmocote type fertilizer once a month (because I have to water so heavily everyday), and a drink of liquid fertilizer like Fish Emulsion occasionally too. That will make a difference in fruit size.

And finally, the first flush of fruits is nearly always bigger than successive ones.

This year I grew an Apple pepper in a container and two out in the garden. The fruits on the container plant were about 30 percent smaller than those from the garden. I think containers have a dwarfing effect on peppers.

My pots have diameter of roughly 2 feet and they have a slow release fertilizer. But I am thinking that probably it is the containers dwarfing them.

I have grown peppers before, but not recently and not these varieties. It could also be that they are just smaller. All of my habanero peppers are in containers, but they seem to be normal sized. Of course they are smaller than bell peppers anyway, but that makes me wonder if King Arthur and Red Knight just grow smaller fruits.

One thing I have not done is added a bit of sulfur to the soil. Some people say that if you add a few match heads to the soil, that will make the peppers larger. Others say it won't. I've never tried it, but I may have to go ahead and experiment that way for next year.

Our bell peppers and jalapeno peppers were small. I attributed this to too cool temps at night (~40). We went from winter to hard/hot summer in just a couple of weeks.

We have had a strange summer in the D.C. Area too. Much of August has seemed more like autumn than summer. This could also be part of my smallish peppers this year too. My habaneros are doing well at setting fruit and it seems normal sized, but I am worried that the cool weather may cause them not to be very hot this year.

Just for discussion's sake, I am growing bells this year in pots (same variety) after growing them in the ground the previous two years. I have seen very little difference in size of the fruit, but the plants seem a bit smaller. We too have had a cooler than average summer, but temps have still been in the high eighties since mid June. I have two Big Bertha plants and have frozen two gallons of chopped peppers just from them. They are loading up for another picking later. (I let them all mature to red unless I just need to pick a green one here and there.)