Originally posted by ChrisJ
It's because the DNG Raw format doesn't have any compression, the PEF and all other Raw formats do. On a Q the file size is always 20Mb for a Raw file, the PEF files will all be different sizes depending on content, those with lots of fussy detail will be larger than those with just a few continuous tones.
This is mostly wrong. The Q only supports DNG, so PEF is irrelevant. Modern Pentax DSLR cameras support compression on both DNG and PEF (and in some very modern instances don't support PEF). The file size of any RAW file, regardless of format, will always vary even if the RAW data is uncompressed, because there is an embedded JPEG, the size of which varies. On the Q the RAW data is uncompressed, and the JPEG data is whatever quality you set for JPEGs. (Unlike the K-5, where the JPEG data in the RAW is always crap quality.)
And I'd already posted the Q-relevant parts of this in the post right before yours too.