Why is the Fuji RAW file bigger than Sony and Olympus?

Amnon G wrote:

Browsing through DPReview image quality comparison pages I noticed that with same resolution files, the Sony NEX-6, MEX-5N and Olympus OM-D EM-5 are about 14MB-16MB whereas Fuji X-E1 RAW files are about 25MB.

What's the reason for that?

Potential reasons (without knowing):

1. More bits per pixel in RAW format (the Fuji would have to have 66% more bits, e.g. 12 vs 20 or 8 vs 13, which is obviously not the case)
2. Better lossless compression (aren't all using pretty much the same lossless Lempel-Ziv compression?)
3. Lossy compression (I remember at least the Nikon D90 threw away some RAW data compared to the more professional models, don't remember the details).

Anyone knows for sure?