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.
Much like a McDonald’s burger, they always look much better in the ad photo. But yours look perfectly healthy, if small, so I don’t think you did anything necessarily wrong, but if you started late that could certainly be a factor.
Iirc we bought these around June July-ish? So probably on the late side, could try to repot them in a bigger pot next year and see if they grow any bigger.