How do bonsai leaves stay small?

Bonsai leaves become smaller, and stay small, as a result of regular pruning, and pinching off of leaves. plus the fact, that they are containerized.

The pruning and pinching, causes the tree to develop ramification of the branches, i.e more twigs, more buds, therefore more leaves.

So where you once had e.g one primary branch four secondary branches, and 8 tertiary branches with lets say 20 leaves.
You now have, one primary branch 8 secondary branches, sixteen tertiary branches, and something like 60 leaves.

Because you now have all these leaves, they have no need to grow as big, because the surface area that receives sunlight i.e creates photosynthesis, balances out with what were once larger, but far fewer leaves.

I hope my answer is sufficient, and helps to solve the query of small leaves on bonsai..
P.C.Barrow U.K