I’m trying to figure out why they are so small a...

I would like to add further evidence that your symptoms might be caused by having too small a container. Notice that the big one has the most noticeable symptoms. That could mean that it's getting rootbound before the smaller ones. Try carefully transplanting into a larger container and see if symptoms subside. Be careful to not water too much in the new container because the roots will not be sucking up as much water relative to the new container's size. If you haven't already, make sure you look up appropriate container sizes.

Agree with the others on the watering maybe make sure you have proper drainage holes, but also if your lights are TOO strong, plant leaves will 'shy' away from the light... Move the lights up farther from the plant a few inches..but 1600 watts is strong for those babies.

I'd have to agree with the other guys here dude. Symptoms are consistent with overwatering but with your description and size of the pot it doesn't fit entirely. Also of concern is the yellowing on the lower leaves and the yellow edges on the one leaf closest to the cam. Possibly a nitrogen or potassium deficiency but that doesn't make a whole lot of sense either if you're growing in soil. If you have any other better photos or details on the soil you used (as well as if👊 you amended with perlite) would be helpful to better assess this. If you're confident that the plants haven't been overwatered I would suggest that you look to transplant to a larger container as well. Keep us posted dude and good luck!

Hi there ! Drooping leaves associated with your leaves colours makes me think that your plant is over watered. The weird thing is that based on your "pot" size , I bet you had both over watered your plant than let the soil get too dry before over watering again. The best advice I could give you is to put them in a bigger pot and to use the "pot weight" technique to know the best time to water yout ladies. Hope it will help you 😁👍

You give us almost no information ti work with. So we can only make educated guesses. Are you using good rich soil or something you dug from the back yard? They look over watered; seedlings take really low amounts of water, that's why most of us plant them un solo cups, in big pots is too easy to overwater. You say 3 days between waterings, if you have been saturating the whole pot, that's too soon, let the soil become bone dry before watering again