Lightroom :: Why DNG File Converted From MRAW File Bigger Than One Converted From RAW File

I use Canon 60D and normally shoot in mRAW. I am considering a move to DNG conversion so I did some test conversions and was amazed to find that, far from reducing the size of my mRAW files, it increased them by about 60%. I repeated the exercise with RAW files and found that this did reduce the size of the files by over 10%. In fact, i ended up with the crazy situation where the DNG files converted from mRAW were much bigger than those converted from RAW.
 
Below are some sample sizes based on Medium previews, Embed fast load data - YES, Embed original RAW file - NO, Compatibility Camera Raw 7.1 and later
 
Before :   mRAW       17Mb          mRAW 12Mb
After:           DNG        28.5Mb      DNG  21Mb
 
Before:    RAW          23Mb          RAW 22Mb
After:        DNG          20Mb          DNG  19Mb
 
If I went for Full Preview, the DNG sizes rose by about 2 Mb
 
why the mRAW conversion is so inefficient?
 
I am still not entirely sure whether to adopt DNG but, if i do, it looks like I certainly won't be using mRAW as the source.