Why is my girl so small?

Hi Guys, I’m trying to grow in a really small space (old microwave) and so far this is all the growth I got in little less than 3 weeks. Still looks just like a seedling.

Strain: purple haze autoflower Medium: cococoir 70% perlite 30% Light: around 5 W of LED, a mix of purple and full spectrum white (5000K). Made this from some Led strips as an experiment. Giving her water with a little bit of nutrients from terra aquatica tripart

Do you guys think the light is the only issue here? The first week I thought she was getting enough because the stretch is not that bad. I have been keeping the coco a little moist, never letting it dry out fully. Quality of the coco is very good.