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.
Mine still tasted great, for what it's worth! Chopped em up into noodle soup in the morning, sliced some into some quick pickles or blended em into sauces.