Might be watering them inadequately, missed the peak growing season, which is summer (most likely, though some are still growing) impatient or all of the above but I feel like that's the size they grow, apparently I've read somewhere that they're of the "ornamental" variety which means they're that size when they've matured, but the picture shows a bigger size. They're Thai Dragon Chilies. Unless I wait until next year to see if they grow bigger or to move them to a bigger pot.
I love it for the quick spice, unlike cayenne which come slow and only creep in a bit later, so it's harder to adjust the spiciness. And it also fade quickly too.
Tbh I could opt for a bit more spice, probably could try growing scotch bonnet next year.