Indoor Bag Seed Grow

The largest plant is doing well but I dont get why they it's not growing taller but yet so wide. The medium plant seems to be progressing slowly but healthy and well shaped, and the smaller ones are coming along. I continue to gradually increase my nutrients each weeek then see how they respond. These are under 340 watts of flourescent lighting thats on a 20/4 veg cycle. The oldest plant here is about 28 days from germination. Would like to top soon but want them to get taller. Tips and comments are appreciated. Thanks for looking.