Very slow growth. Half the size of other. Same con...

It'd be helpful to see the other plant you're comparing to.. and more info in general.. First off, Fox Farm Ocean Forest is very nutrient rich soil. Autoflowers normally don't need as much nutrients as regular photoperiod plants - and I've had photoperiod plants that showed symptoms of too much nutrients when in ocean forest.. That makes me think maybe its too hot of a soil for your autos. Also, you have to realize that with most seeds, especially autos - you may be growing the same strain, but each seed is a different expression of those strain genetics. Say you bought a 5 pack of these seeds. A few may be vigorous growers, a few may be runts.. A few may thrive in the soil you chose, where as some may not.. Its unfortunate that breeders and vendors dont make that abundantly clear when you purchase seeds.. but each seed you grow is basically a roll of the dice. As for advice moving forward.. Don't stress too much about it right now.. keep it going as it is. 300 PPFD is perfect for the stage of growth you are at but you will need closer to 500+ as the grow goes on. I personally keep my canopy at 900-1100 PPFD once flower comes around. Good luck