The 4.3MB is the size of the JPG file in bytes. This is not to be confused with the number of megapixels in the image, or the number of megapixels of the camera sensor.
Honestly, I don't even know why they show you this number on the little screen. It's theoretically a very rough indicator of how sharp your focus was, but only in some circumstances.
[ e d @ h a l l e y . c c ] http://www.halley.cc/pix/