Why is my plant so small? How can I promote growth?

roots develop differently in hydro vs solid substrate. it could have slowed it down... every plant has a different rate of growth. if there is more inbreeding involved you can get what is called genetic depression. generally correlates with less vigorous growth. I've had plants that explode and others that just grow slowly. I'm sure some of the time, but not all, i am shooting myself in the foot too. O2, and gasses in general, dissolves almost instantly in water. so, if you think the water is low in O2, agitate it. Stirring until there is a vortex will actually do a much better job oxygenting water than air bubbles from a pump. You do not have to do it for a long period of time... it will reach carrying capacity relative to the increased circulation at the surface in no time. They are not droopy. they don't seem to be drowing, so O2 is likely fine. If you have a 50/50 mix of soil and perlite or 70/30 of coco/perlite (or perlite equivalent), then the roots are fine. allow pots to get at least 50% of wet weight before irrigating. waiting for top to dry will help avoid pests and algea growth on surface. also helps avoid superficial roots. internode lengths seem fine. you can create more stretch, if you like, by raising lights 2-4"... it doesn't take much, so adjust slowly. i would guess it' mostly a transition from hydro to solid substrate. transplant shock will be more common when moving from a water medium or aeroponics to a solid substrate -- soil or soilless. They look mostly happy and perky. there's no o2 issue.