Welcome to RFF, Jeff! I'm a recent convert to the Contax G2 as well, and getting to like it a lot. One of its features I'm sure you know is a zooming viewfinder that is keyed to zoom to a certain spot by the presense of different lenses. Because of this zooming ability, it can also zoom very slightly as the lens is focused to present the user with framing very nearly correct for the lens... at the focused distance.
As the lens focuses, there are two factors that call for correction: One is parallax, which is not really correctable, but more like "compensated" and correct only for objects at the focus distance. Things closer and beyond will have a slightly different relationship with the subject on film than you see in the VF. This is simply due to the fact the lens and the VF see the subject from slightly different positions. Secondly, there is field size... As the lens focuses closer its effective focal length gets longer, by the proportion of its movement away from the film. So as the lens moves further from the film to focus, its field size shrinks a little, apart from the subject looking larger simply due to being closer! And its this field-size correction of the zooming viewfinder that you're seeing. At some times, the subject is closer than the previous subject, and on other occasions, it's farther, so the zooming will go in different directions depending on what was focused on the previous time. Make any sense? 🙂