The sRAW (small RAW) format is gaining popularity among 20+ megapixel camera owners that occasionally don’t want as much resolution or such a large file size. sRAW files are half to a quarter of the resolution of a full RAW and this equates to smaller storage requirements and faster processing for projects that don’t require as much resolution. sRAW files do maintain a nearly all the development flexibility of a true RAW file, and therefore are often more desirable than a less flexible JPEG.
sRAW files aren’t actually true RAW files. The sensor’s true RAW data is demosaiced and rendered to a lower resolution file and saved in the YCC color mode. For this reason the final file size isn’t quite as small as one might hope (IE: quarter resolution sRAW files are about half the file size of a full RAW file). Unfortunately, when these files are currently converted to DNG, images are converted to less efficient RGB space and the file size increases – negating some of the file size and processing benefits.
The chart adove illustrates the file sizes of a full res (resolution) raw file, a half res sRAW1 file, a quarter res sRAW 2 file before and after conversion to DNG, and a full res large JPEG.
Perhaps the DNG file format will be updated to include YCC sRAW files at their smaller file size. In the meantime, if you wish to keep file sizes small when shooting sRAW, I recommend sticking with the proprietary sRAW format that your camera uses (CR2, NEF, etc).