As a matter of interest Robert - are you embedding the original in the mRaw DNG conversion?
I've just converted an mRaw (on export, not import - I'll try that in a mo) and it's 16 mb, within a couple of kb of the original.
This with "Embed Fast Load Data" unticked.
Another file, Copied as DNG on import - 16.4 mb, within 1 kb of the original.
I went and checked the preferences for File Handling, and for DNG conversion, I did not have embed original raw file checked. Fast Load Data was on. Should that have much of an effect on file size? I'll try an mRaw import with that being on and see what happens.
Canon sRaw and mRaw formats are complex beasts, and not really raw any longer. What Canon do is "chroma subsampling", which basically means that the data is transformed to luma plus chroma, and then some proportion of the chroma information is thrown away. AKA, you have luma information for every pixel in the image, but chroma information only for every second or third pixel - the ratio varies depending on whether it's sRaw or mRaw. (I simplify a lot, btw. Google chroma subsampling if you want the actual details). So you now have a much smaller file, without much loss of apparent detail, because the human eye is much more sensitive to luma detail than chroma detail.
The problem for DNG conversion is that the DNG specification does not include any way to encode Canon-style chroma subsampling, so to convert to DNG, the information needs to be converted to a "normal" image, which is much bigger. What DNG can do is other forms of compression, notably lossless. So the final size is the result of expansion due to the reversal of the chroma subsampling, and the compression by whatever DNG compression options you have set. End result is that the file can be anywhere from much bigger to somewhat smaller than the original, depending on those options and the nature of the image.
That is a great explanation! In my experiment with embed fast load data vs not, it made very little difference. I would prefer to use DNG going forward, and since, for my situation, the mRaw conversion produces a larger file, I see no reason to continue to use mRaw. I was only using it to save HDD space anyway.