Auto period plants are naturally more short and stout. You can fill up the space more by low stress training them for more main tops since they will only get so tall anyway. Not really doing anything wrong since the plant looks happy!
You are not doing anything wrong its all about the bean.
Autoflowers are for outdoors in shit part of the world, otherwise i dont get the point of them.
I got really dissapointed, have more than 100 of them grown and maybe 5 where bigger than 100 g , even outdoors with good summer and early start indoors.
What is those white stains on leaves, powdery mildew ?
She will be still nice !
Only bigger pot would make better result if you would start second chance, strange that even with co2 she is small.
And 1 liter a day for such small pot and plant is too much.
Good luck !