The D100 produces just over 6 megapixels which yields a RAW file of about 9.7 MB, allowing 107 shots per 1GB microdrive. The 9.7 MB file size has made sense to me intuitively since there are 6+ million pixels and 12 bits per pixel, plus some formatting and EXIF information.
I'm curious how the 1DS fits 11 megapixels of RAW data in a file size that is just slightly larger than the D100's. The D100 also has a compressed RAW mode but that slows down the file write times dramatically so I don't use it.
The D100 compressed raw file is 4.5 to 5 MB. Yes, it is slow to store, but if you are shooting landscapes, for example, that may not be a consideration. You can decide between compressed and uncompressed raw depending on how many files you want to squeeze onto a card and how fast you think you will be shooting. It is easy to change from compressed to uncompressed mode.