Hi. I must say I have never achieved pencil thick leeks before I put them out, and I think that’s common. Yours still sound a bit small perhaps but do they have good length leaves and stem? If they do you could plant them out. I have only used the method though of dibbing holes and putting separate leeks into each, then watering them in without filling holes with soil.
If they don’t have good length I wonder about the depth of your modules? They may need planting on, or out to allow theem to grow and spread their roots