Smaller speakers provide a much tighter response due to the smaller cones and have a much better high frequency range. They can still produce a good good bass response, you just need more of them to push air. Most bass players nowadays prefer 10 inch speakers to 15 inch. You'll see many bass player using 4 x 10 cabinets. The main advantage of 15 inch speakers is that they throw the bass frequncies over a longer area. But for a lot of rooms multiple 10 inch speakers is the way to go.

I use a powered cabinet with a 15 inch and a large horn with my keyboard but I do notice a loss of upper mid frequencies and I am seriously considering changing to a smaller speaker format.