Can someone help me figure out, why my plant is so...

You are right, it's too short for 6 weeks of age. My best guess: light was too close therefore plant couldn't grow taller. All other parameters seems to be within norm and your plant looks healthy.

I am definitely no expert - quite the opposite, I’m a complete beginner! But I’m 7 weeks into my first grow and in week 4 and 5 my plants were still fairly small and my expectations were pretty low. Then all of a sudden within a week or so they easily doubled in size. I was having to adjust the lights daily by about 10cm ish. I can’t offer advice but I had the same worries as you and now the plants are so much bigger than I ever expected. Good luck with them :)

El estiramiento de la planta es causado por la luz. Su planta tiene sufiente luz que no le hace falta estirarse para conseguirla. Si desea que su planta se estire un poco solo baje la potencia de la luz o suba su panel. Aunque no recomendaría hacer esto. Ya que su planta se estirara lo suficientemente en etapa de floracion.