I would never buy these fast growing conifers in tubs, given the choice. I also look for magnolias in small sizes. For instance I just bought a 'Bracken's Brown Beauty' in a one gallon pot at a local independent retailer a few days ago.
Production time increments between one size category of a particular tree or shrub and the next size category are typically short - say one year in many instances. Yet the pricing often jumps up markedly each time. So fast growing kinds of conifers in 15 gallon tubs are probably only a few years old. The faster wholesale growers can get plants to grow the more often they can sell a crop and the more money they make.
Q