As others have asked, did you run the calibration software on your receiver? Also, do you have a sub? As others have also mentioned, your AVR (Audio Video Receiver) won't send frequencies to your L/C/R and surrounds that are above the crossover freq (usually defaulted at 80hz) for most AVRs when in surround mode. When running 2-ch, your AVR may be sending full range to your speakers (mine won't however).
As sdifox mentioned, properly setting up a system makes much more of a difference than speaker size. Hell, speaker size really doesn't make too much of a difference at all (for our purposes) unless we are talking about LFE (i.e. < 120hz).
I suggest you read this:
http://forum.blu-ray.com/receivers/38765-calibrating-your-audio-spl-meter.html After you run your calibration software, follow the instructions in that link. I would suggest you read it over multiple times to get an understanding on what you are doing.
I also suggest you pour over the message boards on that entire site. Read all the stickies made by Big Daddy. You will learn more crap you ever thought was possible when dealing with a home theater system. Also, buy an SPL. You can't properly calibrate your system without one!
I see systems all the time that are setup like crap. The box stores trick people into thinking they can just buy speakers, subs, and AVRs and just throw them into a room and they will sound great. However, it takes proper calibration, room treatments, and quality products to really blow the socks off people. Anything else is just (loud) noise...