OK, I'm a relative newbie in this whole XC thing, but I had to do some similar Checkout redesign work on my own site yesterday.
I looked at your CSS with FF and Firebug and there are two lines that you can take out to fix the display problem. Open main.css, go to line 4576 and delete two properties:
* padding-left: 30px !important;
* width: 850px !important;
Caveat: obviously someone put that "!important" bit in for a reason, but I didn't see any unintended consequences (in FF4 at least). So test it thoroughly. YMMV, etc.
Good luck!