100000x100000 makes 10,000,000,000 pixels.
24 bits per pixel (16.x million colors) would result in a filesize of about 27 gigabytes.
16 bpp would result in a filesize of 18 gigabytes.
8 bpp (256 color) would be about 9 gigabytes.
4 bpp (16 color) would be about 4.5 gigabytes.
2bpp (4 color) would be about 2.25 gigabytes.
1bpp (b&w) would be about 1 gigabyte.
Based on the filesize you mention (10 gigabytes) it's probably 8bpp, either 256 color or 256 grayscale shades. I looked around briefly at the site you linked to, and didn't find metadata that would give me any further insight into the images you found.
I use some pretty big datasets on occasion in geospatial analysis, but I'm usually working on smaller scale (state size, more often county or sub-county). This thing is enormous.
Jim