Yeah, I just bought a 2002 RS the other day, and I have to say this is the first car I've owned that makes me feel fat because of the seat width. I'm 5'9 and 175 lbs.. not what I would call huge.. but the seat width does feel really skinny.
It's not to the point where it's uncomfortable, but I certainly wouldn't complain about an extra inch or two in width.
If you're really concerned about it, order a set of JDM front seats and install them in your STi.