You need to think of leaf reduction as a series of ratios and balances.
Firstly your tree has a balance between the root growth and the canaopy growth. When trees are growing fast and from a young age you will see a trunk that will shoot up prior to it even throwing its first branch, then when it does throw some branches these will be few and will themselves want to shoot upwards. Leaves tend to be fewer but largish. The root system at this stage is following the same pattern.
As your trees roots find some happy boundaries and then start the process of producing finer feeder roots back towards the base, then the canopy will start displaying the same characteristics of branching etc. This process takes time, however as clever little bonsai people we are we can alter and fudge this natural process with tecniques such as potting into smaller pots, trimming roots, cutting back tunks or branches to induce ramification etc.
When a trees root system is constrained, the canopy will carry by default the amount of leaves it needs to live in harmony with the root system, no more, no less. Lets say your tree has only 2 branches, each with 2 large leaves. If you cut this back to now allow each branch to produce 2 more branches, you now have 4 branches with 8 leaves. Going from 4 leaves to 8, the leaves need only be half the size to support the root system. This is perfect for the ramification we need but applies to bonsai that are in their finished pot as this process slows down growth rates of the tree. If you want fast growth rates to produce a fat trunk, then you need roots that are not constrained, which means roots that can grow can support more or larger leaves, hence why you can't 'grow' a tree and nicely reduce the leaves a tree at the same time!
In essence, if you are growing your trunk and branches still, then don't worry about leaf size! The more branches you start putting on and then the more you start to manipulate these the leaf size will naturally start coming down.
