What makes speaker's sound big?

Hello Martin. I've always had big speakers as my listening room is rather large. I've just changed to smaller ProAc speakers on sturdy heavy stands and added two 15" sub-woofers. One on each sides. The sound is pretty big and it will fill the room quite easily. I think the only draw backs will be the SPL your small speakers will deliver. If your trying to have a rather large and loud party, this set up will probably not do as usually, the power is limited with smaller woofers. The good part is that the small footprint your bookshelf speakers will have, is going to give you a better musical stage definition. 
The previous speakers where OBs. The thing I miss the most is how quickly they reacted to the music. But I do not miss the added space they took in my listening room (living room). I will have to put them up for sale as I rather like the added space that I get with the small speakers. You can't win them all.

Big Speaker:)

the answer is yes, you need a big speaker for big sound, while you can produce a reasonable sound stage out of a smaller speaker there will be no impact or force from a mini speaker. Mini or small speakers will always sound mini or small, its just physics and anything else is just a illusion or delusion or whatever you want to call it.
A big planar speaker with a big surface area will sound bigger, an array of dynamic speakers emulating a planar speaker will sound bigger. If you combine a big sub woofer with tiny speakers you will get impact from the subwoofer but the rest of the musical spectrum will still sound small - like the small speaker where the sound comes from. Same like the other poster said: a big block engine will always sound bigger than a tiny 4 cylinder engine, even blown up to deliver hundreds of horses, if you want big than go big - and yes again bigger is better as long you have room!

well sir ,  you have good hearing  selling the speakers  i was happy to read that. Then you said 
B&W.  Keep looking about.  Speaker wire , Amp,
 Pre, all work to fix the problem.  i don't know how far from the wall the speakers can be  so i won't comment   

This is what happens , Some expert will say,,,
,,Ribbons and AMTs that spread around sound will sound bigger than a single dome tweeter.    The top two the Professional talks 
about need to be a way from the wall behind the Speakers.  start at 40 in  out  then move around  
 the 3rd tweeter is the one
you might be interested (example) Merlin VSM BM  
but again never sure  good
luck