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.
You're absolutely right, they go great with Pak Kra Pao, though it depends on your spice tolerance, I like mine to be on the more spicier side.
They do look normal to me and healthy so I'm probably doing something right, thank you.