Suggestion...

Before you purchase a larger suitcase, what about throwing a duffle bag into your regular suitcase? On your way home, you can put your laundry in the duffle bag and still have the better structure of a real suitcase for anything you pick up in Europe.

I travel to a few cities during the week and always have the spare bag just in case I pick things up along the way. (I also FedEx a box of laundry home during the week if I'm gone for more than five days. Not sure what it would cost to ship a box home from Europe, though.)