On my previous camera bodies I had occasionally shot at very high ISO settings as a result of the lack of light.  I had not paid a huge amount of attention to any secondary effects of doing so.  My current cameras had a work out in some very low light when I decided to test them in some rather unfriendly conditions.  When I was at home, I was running some disc backups and I found I could not get the normal number of files onto a single disc.  A quick bit of investigation and I could see why.  The high ISO shots had a significant increase in file size.  As I understand it, RAW files, while containing all of the data from the sensor, do have an amount of compression applied.  I imagine that the noise inherent in high ISO shots means that the compression is less effective as there is so much variation across pixels.  As an example, a shot at ISO 320 will average at about 22Mb.  The shots at ISO 51,200 are coming in at over 30Mb.  At ISO 204,000, the files can hit 40Mb. That is quite an increase!  Something to keep in mind when planning to shoot in very low light conditions.