Also I was wondering if smaller sensor meant better ISO performance? Or is it just better technology? I'm wondering that since I've noticed 400 ISO on the GH1 can equal ISO 2000 on the 5D. If we can figure out why then we can tell either Canon or Panasonic for future cameras.
Ok first of all we all knew this already it's not a secret. The GH1's larger sensor is used to shoot photos @ different aspect ratios without compromising FOV (measured diagonally.) It is slightly larger than the image circle, and the corners get clipped, so you're never taking anything at full size of sensor, and taking photos @ 4:3 it's the same as the G1 sensor, but since the sensor is wider you can do 3:2 and 16:9 and gain some extra space on the sides, without just cropping it out of a 4:3 sensor or image.
From DPreview
Secondly on 4/3": The name of the system comes from the size type of the image sensor used in the cameras. The image sensor is commonly referred to as a 4/3" type or 4/3 type sensor. The common inch-based sizing system is derived from vacuum image-sensing video camera tubes, which are now obsolete. The imaging area of a Four Thirds sensor is equal to that of a video camera tube of 4/3" diameter.