It's what called the "shrink" method, which allows you to adjust aspect ratio without adjusting your lens. The downside to this is that resolution in 16:9 and 4:3 content is reduced (16:9 is about 1440x810 for example) as well as not as bright. It is however the most consistent in terms of brightness with the zoomed 2.35:1 CIH image.
Another factor the RS45 lacks is that there is no real color management system (CMS) that I recall (the RS25 does anyway), but since you only used pre-calibrated modes, this is kind of a moot point.
I wouldn't simply recommend a lumagen processor for their price, despite having excellent aspect ratio controls, simply to use the "shrink" method, although the CMS controls are more sophisticated than on the RS25. However, I'm more of a HTPC person these days, which the lumagen wins for convenience.
It's really how much you want the extra brightness and 3D is a factor to going from the RS25 to a RS45. The RS25 looks better than most consumer grade projectors out there with 3D.
Hopes this helps.