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.
I've had a few ornamental peppers that were exactly that size. And you're right, the package pic does not match. That may be down to the marketing people getting the wrong pic off the internet, your seeds being a slightly different cultivar, 🤷🏻♂️. Even us pepper nuts get stuff mixed up and crossbred.
That plant just screams healthy. My first thought was to feed it more, but you must be doing that to get those results. For next year I'd get the seeds from another outfit. I almost always use eBay.