Bigger speakers could actually make your problems worse!
There's nothing wrong with the speakers you have, aside from being weak in the extreme low end. But that's
not the cause of your problems. It's trying to reproduce bass frequencies accurately in a 12x13 room - which the laws of physics make damn near impossible, even with gobs of bass trapping.
What you
can do is take some objective measurements. The easiest one is to grab a copy of Ethan Winer's stepped sine project. Set up a mic (preferably omni) at the listening position and record the project as it plays. This will tell you in 5 minutes which frequencies are the biggest problems.
Even if you can't get rid of the problem, there is tremendous value in just knowing specifically which frequencies the room is lying about.