I agree. But since it (mostly) isn't compressed, it will give you a good indication of how much memory you will need
BMP uses RLE (run-time length encoding), which can actually be very efficient for images that use relatively few colors.
If you're talking about true-color photo's then yes, BMP is doesn't compress much... but the less actual colors you use, the better the compression gets. BMP filesize is not a good indication on how much actual memory the decompressed image would take up.