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 grown Thai ornamental Chilis in pots and in the ground, and they have always turned out small like that. Probably 1” to 1.5” long at the most if I recall correctly. This year we did a purple/black ornamental and they came in about the 1” long but more bulbous.
Those purple/black ones I used to grow a lot. A version called Gusto purple. They might be small and folks think they're pretty but they're also quite tasty. And if you give the roots some room they're prolific.