So? Did some event occur in the last century or so that suddenly means unifying in this day and age doesn't increase power? Furthermore, Those countries aren't "tiny" by any stretch of the imagination.
By your logic, the US conquering Mexico and Canada would only make it stronger, when one can pretty safely assert that the opposite would be the case; the resources being invested would not remotely be offset by the gains of the territories. In the cases you cite, there was in some sense a nationalist drive behind them that made unification something more than simple territorial acquisition - it was so powerful that even German intellectuals, who chafed at the idea of a Germany dominated by authoritarian, militaristic Prussia, still bought in to the idea of a unified Germany. It was the union of like-minded areas that basically were prepared to submit to the greater union instead of their own regional interests.
It seems reasonable therefore that dropping an unproductive or dissenting region, one that is unprepared to actually undergo such integration, may well improve the state of a nation by freeing up resources that can better be spent elsewhere. Losing thirty million social and political atavists and a financial black hole seems at least somewhat likely to have this effect in the United States, streamlining governance and economic processes, dramatically reducing financial burdens, and allowing for a leaner machine overall, probably without significantly reducing its capacity to completely destroy the world if it really wanted to.
