This means that if you set the compression to be very gentle, it can easily still be larger than the raw. It is the uncompressed bitmap size that is the base size for the compression to work its way down from, not the raw size. Now to the point, since we are talking about jpegs here. So you can expect an uncompressed 16bit RGB "bitmap" to be approaching 3 times larger than the raw, and times 3 divided by 2 for 8bit uncompressed. Nikon NEFs store full res previews while Canon uses half res, hence the larger fraction.
Uncompressed RGB files (3 values per pixel) will be larger than your raws, as the raws contain a monochrome bitmap (1 value per pixel), and usually a downscaled, aggressively compressed preview that takes a fraction of the size, 400k for Canon 10MP cameras, and 1M for Nikon D5100 (I know these numbers because I used to read them out of the raw files and store them in a temporary file).
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