Reading in between the lines, it's possible to store them in the same pixel resolution they were taken in and compress the heck out of the image. So, do we know if they compress the file size even if you're paying for "Original" backup?
That is a valid concern, given that images <16MP are still recompressed when uploading in "high quality" - but I don't think that they do this to "original quality" uploads. Paid space is paid space, after all. :)
I just tested this:
- take a high ISO image in a dark room while image qualities are set to high, resulting in basically black images with a fair amount of noise (~2.5MB JPG, ~22.5MB RAW @ 20MP). These images should be fairly easy to recompress, because none of the noise in the lower bits is really visible.
- set Google Photos to "Original quality", upload images.
- set Google Photos to "High quality". At this time I'm asked if I want to recompress formerly uploaded images, which I decline.
- download images again. The duplicates have the same size in bytes as the originals.
Recompressing images could still be done at a later point, but I doubt that this is the case.