In the size department the Olympus PEN clearly wins. Being nearly perfectly rectangular its profile is a lot smaller than the G1, and even if the G1 was smaller the way you perceive the PEN is that it's a very small camera, just by the way it looks. This doesn't mean the PEN feels cramped though, the button layout is well thought out and overall it's decent to use.
Here is where things start to get dicey. The G1 screen itself essentially blows the PEN screen out of the water then kicks it in the face. First, the screen resolution, the G1 has a higher resolution screen (460k vs 230k). Secondly, outside in daylight the G1 screen is much more viewable than the PEN screen, and the fact that it articulates just brings in a new dimension of shooting. Oh and not to mention the built in electronic viewfinder on the G1 is insanely nice, rendering sharp details and making it ideal for manual focusing, especially with the manual focus assist which zooms into the focus point for more precise focusing (in good daylight of course).