First off if you think you need a big size pot to grow a small tree go to this thread and read up. The picture is in the 4th post down.
https://www.rollitup.org/indoor-growing/111514-need-lots-info.html
From there you must understand that you are switching pots before the plant gets root bound. Just because the pot is full of roots doesn't mean it's rootbound, when the pot is too small it is effecting growth then it's root bound.
Starting out in a small cup gives your plant a chance to grow a nice root ball and helps to increase stem size in veg growth.
You switch to a bigger pot from the cup to the next size pot when you start vegging (about 2 weeks). Going to the next bigger pot gives you new fresh soil and gives the plant new vigor and encourages growth. With the new soil and extra space the plant just wants to take off and grow even better.
You switch to the next size pot when you start to flower. New soil and new space and your plant is encouraged for more growth right when you want it to. You keep a nice root ball all the way thru the flowering stage which helps with plant health.
I can recommend this type of growing and think you should give it a try if you haven't already, it works really well.
You can also just put everything in a 5 gallon bucket when you start and it will still grow fine, but I think you are better off the other way since you are giving it fresh soil and nutrients at key points in the growth cycle.
Good luck with your grow.