I think a more realistic comparison is a BMP image (16 or 256 color) because the game didn't compress the image the way PNG or WebP does. When I converted that tiny ~3KiB image to 16 color BMP the size was 28.1KiB, for 256 color it was 57KiB. But some colors were lost.

Anyways I don't think we can have 100% apple to apple comparison, because the game used a different compression and rendering technique. Also consider the actual amount of RAM / VRAM the images are occupying. In RAM / VRAM they are probably in a decompressed form which is much more memory.

I agree that there's no perfect comparison. I just think that whatever image format you use for the comparison, you should be storing an original picture, not one that has been stretched or otherwise modified.

FWIW, if you convert the 3KB image to 16 colours in GIMP (Image | Mode | Indexed... and choose "Generate Optimum Palette") it looks exactly the same. I'm pretty sure there are only 16 colours in the image. The resulting PNG is 1,991 bytes.