It is if you also want to keep the smaller tank, in which case you'll need to buy an appropriate sized filter for it anyway. And who really wants to sell a tank?! Also, the size of the filter could limit your potential to upgrade, unless you're willing to run an Ultima 4000 on a 55g!
But yeah, you do have a point. It certainly makes sense if you're just getting into things and bigger tanks are on the cards to buy products (not just filters) that will work on larger set ups. But if we're talking about a permanent set up, there will be a level (depending on stock, feeding, water changes etc) after which more filtration simply becomes a waste of money, both in buying the gear and running it.
So unless you have an unlimited budget, I still believe you can over-filter a tank.