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.
Container peppers can be tricky. The only thing I've found that works is constant fertilization with compost tea or fish fertilizer from seedlings to fruiting, but don't overdo it with the water. Let the soil in the container completely dry out before watering, give it a bit of water then, but one big gulp once a week. That seems to work for me.
Lots of people keep the soil hydrated which isn't what all pepper varieties want. These small chilies are also used to hotter climates.