Why should some speakers need a big room?

Thanks everyone for all the comprehensive & fascinating answers (especially lrsky)!
There sure are some smart people out there.
By the way, I did own Appogee Stages a decade or so ago, and thought there were fine nearfield, but I was fearful of moving on to Maggies or Soundlabs because of the size of my room and always wondered how they would have performed. Thanks again.