bit baffled by this
the 5D shoots at 12.8MP, 2912 x 4368 (I think)
when I convert from RAW, I get 36MB files at 8 Bit. I thought these would be well over 50MB, as the 350D produces 48MB files.
Seems silly upsizing for Alamy having sold a kidney to buy a hi-res camera!

what am I missing?

Presuming you have converted to a TIFF file, have you created Lossless compressed TIFF files perhaps?
Also, it depends on what is contained within the picture. There's interpolation involved with the conversion from RAW to TIFF, which will always vary results.

just converted in PS and opened the file, therefore not actually saved as anything