In my experience, all peppers are, to some extent, restrained by their environment.
It may just be that they are slow, and, certainly at this stage, I wouldn't be concerned, but I'd also give each a 3-4" pot each, just to make sure that if they want to grow, they have room to.
I'm growing 3 varieties of ox-horn peppers this year, (Palermo, Romano and Thor) as, at least in my climate, they seem far more rewarding than blocky bells.
One further point; if you keep them where they are, then, since you have probably watered out most of the nutrients, I'd look to feed them a very dilute multi-purpose feed soon.