Has the tree been taken out of the pot it is in??. If it has you will know what the root ball is like. If it is still as you bought from the nursery chances are you can reduce it quite a lot.
Firstly knock it out of the pot have a look at the roots, if they are a mass that are going around and under the bottom. Have a look in the centre of the root mass.
If you see the soil full of roots you have a fine root system. You will have no worries reducing it.
My bet is though there will be hardly any roots down the centre, so when you tease out the soil you will have a very reduced what is perceived as a root ball.
So again it will not be a problem to reduce it.
The nursery soil allows for the roots to run down the outside. Not down though the centre. To get the oxygen from the holes at the bottom of the pot.
Being a heavier soil than a free draining Bonsai mix it does not allow the roots to go though the soil, but around it.
Just my take on it, the first thing I do with nursery stock. Is to take it out of the soil it is in and replace it with my mix to acclimatise the plant to a good free draining mix.
Pup
Ps nice pot tree match.
IT'S ARRIVED
taking the top half of trees of since 2005! 
