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

Here are the reasons (I think):

Raw files are a collection of different types of data, One of which is the raw data. The size of this starts out as a constant.

Number of bits x Number of Photosites

Common bit sizes for serious cameras are 12 or 14bit. X-Trans CMOS II for the new X100S states 14bits (actually, the X-Trans page phrases it as 'compatible with 14bit formats' which is a bit suspicious, but anyway) so the calculation would be:

14bits x 16.3MP = 28.5 Million Bytes or 27.2MB

However, I think X-Trans I in XE-1 and X-Pro 1 is only 12bits making the calculation:

12 bits x 16.3MP = 23.3MB.

At this point in time, the IQ has nothing to do with the size because there is no (I)mage, just a fixed quantity of raw data.

This raw data could be compressed using lossless compression to reduce the file size without any impact on the data, potentially by a factor of 3 but more commonly by a factor of 1.5 to 2. However, it requires a reasonably meaty processor to do this both well and quickly and some cameras don't bother with this.

In addition to the raw data, the raw file contains other data; several sizes of embedded JPEG previews, a colour profile, Regular EXIF data, and custom EXIF data like Maker Notes. And finally a file has format (protocol) used to described how and where all this data gets stored within the file.

The size of the embedded JPEG previews varies based on how large the camera decides to make them and what quality setting it uses for the JPEG (lossy) compression. This size is affected by both the scene and the IQ and the ISO. More detail/noise = less compression. Typically the embedded JPEGs will take between 0.5MB to 3MB depending on these factors.

So my reckoning on this is that the RAF's raw file contains 23.3MB of uncompressed 12bit raw data, and 2-3MBs of embedded previews, colour profile and EXIF.

-Najinsky