From this page: http://www.mamiyaleaf.com/sample_images.html
I went here: http://www.mamiyaleaf.com/samples_aptus-II_8.html
and downloaded the second Harley cycle image.
That "Aptus II-8" mos file gives me DNG files that are just a few K smaller in size.
Then I downloaded an "Aptus II-5" file (the one with a lime and other test objects). That mos file is 47.36 MB, and when I convert it to DNG I get 29.14 MB, which is about a 40% reduction. Not your 50% reduction, but much more significant than other raw files.
My primary experience is with Canon CR2 files from several models. I get an average of 15% to 20% reduction when converting to DNG. Not the 50% range that Sandymc mentions. I think most, if not all cameras compress their in-camera raw files. Certainly the method and degree of raw compression varies by camera maker. That would account for big variances in compression that different users see.
So, a make or model of one camera might use weaker compression than other makes or models. That apperas to be the case with Leaf. The mos file from the Aptus II-5 model has pixel dimensions of 5336 X 4000 and is 47.48MB. The Aptus II-8 file has pixel dimensions of 7304 X 5466 and is 43.04 MB. Much bigger image, but a smaller mos file.
What is the camera model of the mos file you are using? Maybe consider uploading it to Dropbox or somewhere and we can test it for you.