I think it's because of extra rooting area. I find that tomatoes really take off when I transplant them into a deeper pot. I usually seed them in a 32 cell deep rooting tray and then transplant them into 4.5" pots and/or 1 gallon depending on the space I have at that time. Since we don't plant our tomatoes out until late May, the 4.5" have to go into 2 gallons by late April
