Thanks Bill, that makes perfect sense! If they were all set to large, the receiver would send everything everywhere, there'd be no need for a crossover (and probably a lot of interference...)
I'm guessing that once I told the receiver that I had a subwoofer, it automatically set the rest of the speakers to small (since it's brighter than I am .

Thanks for the tip - I'll play around between 40 and 80, see what works!