If you row mj to 2ft in veg, it could easily hit 4ft by the end of its life, getting the light to the bottom 2ft of that big plant is hard to do (unless you keep adding lights) so if you grow a plant to 1ft in veg, then flower, it will be over 2ft when done, a 2ft plant can yield quite a bit. Thats why "we" usually only veg for a 3-4 weeks (or so, I'd like to grow a monster someday).