thanks for getting back on the point of my question! I can't be certain that they are from the same mother, I bought them from a med clone guy and didn't ask him... (again my bad) but as they looked the same when I got them I just assumed they were the same. .
Well, that may well be the answer then. There should be no reason at all why two identical clones taken from the same mother plant and grown in the same environment should grow differently to each other unless they aren't clones from the same mother. Clones are exactly that - identical genetic replicas of the mother plant that will grow in the same way given the same environment.
I suspect you may have two clones from different mothers which may explain why they're growing differently to each other in which case - treat them differently.
A plant becomes rootbound when it basically outgrows the size of the pot its in. For a plant to grow healthily in a pot, it's root mass must be allowed to grow in proportion to the size of the vegetative growth going on above the soil, as the roots are the plants primary source of water and nutrients that enable photosynthesis to take place if the root zone cannot supply sufficient water and nutrients to the rest of the plant - slow and stunted growth is the result.
It does take a while for this to happen though and when we go through our 'potting up' routine, we're effectively allowing the plant to become 'semi-rootbound' before we pot it up into a larger size and give the well developed root ball more space to grow and fill out. Root growth slows in flowering as the plants energy switches to floral production and that's why the 'potting up routine' ideally needs to take place in vegetative growth - so the root ball is fully formed before flowering starts. The ideal potting up routine for clones is 2-3 weeks rooting in a root cube > repot into 4 inch pots for 2-3 weeks > repot into 8 inch pots for 2-3 weeks > repot into 12 inch pots for 1-2 weeks then flowering.
Each stage allows the root ball to fully form before potting up into the next size and this also has the effect of giving the plant another fresh set of soil and nutrients to use before we need to start feeding.