The size of the pot chosen doesn't ALWAYS affect a plant's potential initially (as appropriate pot size is primarily a function of soil choice), but it has a huge impact eventually. Figs in the ground might need very little fertilizer, but figs in pots are voracious feeders and need regular fertilizing if they're to be at their best.
I'm intimately familiar with the increasingly negative effects of progressive root congestion, and equally familiar with how a lack of adequate nutrition affects a plant's opportunity to realize as much of its genetic potential as possible. This allows me to say that no tree restricted to a 2 gallon pot and grown under conventional container culture could be said to be thriving for even 5 years, let alone 15 yrs, and it further defies reason to suggest it might have remained a thriving tree in spite of it having rec'd no nutritional supplementation.
The term "thriving", when used to describe a plant's state of vitality, means the plant is relatively free of stress and is growing at or near it's genetic potential. This isn't just an abstract idea, it's the reason behind why nursery ops regularly pot up plants, preferably before the soil/root mass an be lifted from the pot intact. Nursery plants are too often potted up after roots have reached a stage of root congestion where the planting should be culled (soil/root mass can be lifted from the pot intact) because it represents a problem tree if planted out.
Trees regularly fail even when planted out in the landscape because most growers are still unaware of how important rootwork is to ANY tree planted out and to all trees grown under conventional container culture, long term.
Al
