It's generally genetic (seed selection/type).

High K can cause thinner walls in peppers. Good leaf mass on the plant prior to fruiting tends to increase wall thickness in peppers.

I agree that it's probably just your choice of varieties. Some peppers are much thicker walled than others.

These Pimentos I grew from seed. They are the thickest walled peppers I know of and possibly the sweetest too. The variety is called "Pritavit".
-Mark
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I like those, Mark. Nice job .

They have lots of leaves. So many, it's hard to see the peppers. Where did you get your seeds, Mark?

Pritavit is sold by Osborne seed co., and probably others too. It's an F1 hybrid so I can't save seeds unfortunately.

Thanks! I'll check them out.