Two Great Reasons:
1. Avoid Root Rot. When watering a seedling or clone in a large container, you may end up with residual water in areas of the container that yet to have roots. If there is organic matter in these areas, and low oxygen due to too much water or soil compaction, you will provide an environment ripe for root rotting fungi to thrive (pithium is a common one). Once the plants roots grow to reach this area, they can become infected by the bad-fungi, and then your plant will have a slow, sad death. This is not conjecture and can be confirmed in horticultural literature.
2. Speeding growth. This one is based on both conjecture and experience. I have seen (hundreds...thousands? haha) of instances where the same age plants were transplanted in pots of differing sizes. The less of a pot size jump, the quicker and more robust the plant grew. It has to do roots hitting the sides/bottom of the pot, signalling the plant that there is no more distance for the tap and feeder roots to search for food, so it must produce more lateral roots closer to the plant. This flush of root growth is coupled with a flush of shoot growth. As above, so below.
Try this sometime. Plant a clone (plug w/seed start) in a 5 gallon and a 4" pot. Pot up the 4" to a gallon, then to a 3 gallon, then to a 5 (or 7 or 10) gallon. Transplant when you can pull the plant out of the pot and ALL of the soil is held firmly by the roots. I will bet you a BubbleSnake that the transplanted plant will be more robust than the overpotted plant.
Seeds can be started in gallons, as long as you are careful to only water what the plant can reach as it is growing into it's home.
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Keep Preaching .....that's what I needed to know .....I want to start another few going and wanted to be sure cause I am always thinking that I am going to screw it up....growing weed is not easy like some people claim it is....but it's great having people like your self and everyone else here helping beginners....thanks Man...