Multiplying like rabbits

I think our belongings might be breeding. Allow me to explain. A couple times a year, for the past several years, John and I have lugged anything from boxes full of random stuff to huge pieces of furniture to the Salvation Army. A conservative estimate would be that we've made six such trips since we bought this house in 2004. I'm no mathematician, but doesn't that mean that we should have less stuff than we have now, and more closet/garage/general space? Assuming that's correct, why does it so often feel like there's no room in our house? I'm left to believe that our belongings are multiplying. Anyone? Anyone? Bueller?