Steve,
The additional issue is that a fig moved from 1 gallon to 15 gallon pot isn't going to use the full 15 gallons. So, if you need to move and/or store your figs (winter for example), you've got a tremendous amount of wasted space and effort.
Perhaps your figs grow a lot faster than mine, but the ones I stuck into 5 gallon containers haven't yet filled out that container after a year.
Andrew