"But I think to get a "Big Sound", you need "big speakers"
I agree in general. You need this in particular for the low end. Using subs is a good strategy to lower cost and fit smaller speakers otherwise into a particular room. Large speakers are often not a good choice for smaller rooms in that they are hard to place optimally and imaging, detail and sound stage accuracy can suffer listening to many larger multi driver designs nearfield. So you need to fit the system to the room and make sure the amp is able to drive the speakers optimally at desired volumes.
