Belgium was invaded because two allied armies were there. And it had been part of France as had the left bank of the Rhine. And it should be remembered that the allies began the war in 1815, not Napoleon. And the Belgians were not too happy to be under Dutch rule, either, which is why they revolted in 1830.

So I would say it was not expansion, but an attempt to regain lost territory which was part of France when Napoleon became head of state.