plastic pots/bags don't breath and force the roots to grow in spirals around the pot/bag (the root is looking for water/nutes and will continue growing around and around).
growbags/smartpots prune the roots - this means that when the growing root-tip grows into the bag slightly, the air causes the root to be 'pruned', this pruning causes the 3"-5" behind it to explode in side growth (similar idea to pruning the top - one cut, multiple heads), filling up the mass of the soil faster with more healthy roots.
since you are using only 1 4" T5 (similar to a 250w-400w HID in power. no depth penetration though), the 1gal bag might be barely enough, but I'd upsize the pot to a 3gal or 5gal to get the most out of her. I just finished up a bunch of OGK and they averaged just under 3oz each in 5gal smartpots @ 40" tall finished