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.
Oof, that guy's chilis became more rounded lol, yeah luckily is the key word here. Still probably taste the same anyway.