What makes speaker's sound big?

These speakers are very efficient (> 98 dB, right?) They have 11 driver elements including a pair of 10" drivers.  So by "big" sound, I wouldn't think the OP is referring to SPL (provided everything is hooked up correctly and we are not trying to rattle the neighbors' windows).

The OP may be referring primarily to the apparent height/width/depth of the soundstage. This is something many of us struggle with.  Before getting into new subs, DSP software, or room treatments, it might be a good idea to experiment more with speaker placement. Several formulas (or procedures) are available on the internet.  

+1 +1 +1    MC give it a break,

dynaudio speakers from contour,confidence  line sounds big.It does not matter if it is  floorstander or bookshelf .

A few years ago, in our prior house, I moved my audio system from our 13'x24' living room to a 24'x26' family room and my then pair of Totem Fire monitors and Velodyne sub woofer with a 10" driver just didn't fill the new room.  I ended up replacing the Totems with the Focal Sopra No2's and the Velodyne with a pair of JL Audio F113V2's and they did fill the room up.

We recently moved into a new home with a 14'x19' listening room and my setup definitely fills the room.  I find I'm setting the volume level on the preamp a few percentage lower.

I auditioned the Focal 1038's driven by a McIntosh MC452 (450 wpc) power amp in a speaker room that was about 22'x22' and was blown away at how they filled the room.  I would have "pulled the trigger" and bought those, but one of my audio buddies let me know that the Sopra line was about to hit our shores, so I waited a few weeks and ordered those.

Easiest answer ever:

MAGNEPAN

I realize that every manufacturer is not making tall speakers--wonder where they got THAT idea??

Try the best and the original.  Jim Winey was a pretty smart engineer, I guess.

Cheers!