Are you, perhaps, seeing the difference between compressed (~100KB) and uncompressed (~12MB)? That is, Powerpoint is reporting the uncompressed size? I know both PSE2 and GraphicConverter report the size "in memory" which is the decompressed full size of the image.
Check to see if, when you save the Powerpoint file, the file size goes up by ~12MB or if it is a smaller increase after adding an image. This would indicate whether or not Powerpoint is re-compressing the images when it saves.