a plant will find and fill its root zone first; before vegging like mad. it has 5 months or so to to its thing in nature. we don't have that much time, and if we do, its to grow a monster.
the plant upon finding the limits of its first home, begins to veg. same with the next pot, and so on. if you plant a seed in a ten gallon pot, and i transplant my way up to one, i will pull ahead rapidly in growth.
you also wind up with a more complex and dense root structure when you transplant. its win-win, and if performed correctly, does not stress plant at all.