When you have a plant at 12/12, that just means you started the plants in flowering mode. That means you are not giving them time to vegetate, but start flowering. If you had a mother, it would be on 16/8 (generally) and all it would do is grow and grow, but will never flower until it gets the signal its fall time which is 12/12.
Your plants do not have time to grow and so they will be smaller when they finish than if you had them at 16/8 for a month (which is typical) before going to 12/12. However, even when in flowering, they will still grow 2 to 3 times the size by the time they finish.
Nutrient burn generally looks like the edges of the leaves are burnt, not the whole leaf. I suspect your yellow leaves are quite natural, since they have been flowering for 2 months, and depending on the strain are getting close to the end of their lifespan. I picture would help in that matter.

keylime