Starting with small pots and moving to successively larger ones serves a very real purpose, and not just to save space. MJ roots grow outward from a central taproot and expand as fast a possible outward. However, that means that the roots are not naturally very dense. Hence why it can grow is arid regions. By starting in a smaller pot, allowing that to condense the root-ball and then transplanting, you create a much denser root system in the end. This means you can use less soil and smaller pots for the final flowering.
Oh, and if you use Superthrive before and after transplant, it will completely prevent root-shock. One drop per gallon. If the roots get damaged by accident while transplanting (still happens to me), use the max dose of 1/4tsp per gallon.
You can get it at walmart or any garden center.