Mike,
<pure semi-informed speculation>
The large part of this is the shortcoming of sRAW. Unlike the Nikon small raw that the D2x produces (not sure what other cameras) which is a true raw image, sRAW is a demosaiced file. This throws out the benefits of raw, where there is only one channel of color data per pixel, and therefore the file can be only one-third of the size.
It's my understanding that sRAW keeps the higher bit depth of raw as compared to JPEG, but it does so by demosaicing and downsizing the file. It also looks like there is some compression (probably lossy) applied to the image.
To make a DNG from this, the data gets expanded to full-res TIFF data, and then saved as a TIFF inside the DNG. If you don't want to recompress the data, you basically have to save as a TIFF, which accounts for the much higher file size.
It's possible that the DNG spec could be updated to simply copy over the image data from sRAW, but of course, applications that work with current DNGs would not necessarily be able to work with the new file in the way that they can work with current DNG files. I'm not convinced that sRAW will be around that long.
I assume Canon can't do what Nikon does because they don't want to pay Nikon licensing fees. But sRAW is a kludge, at best, as I understand it. It probably makes sense for people who want to shoot raw, but don't want the full resolution, to just buy a camera that shoots a smaller file.
Frankly, I think JPEG XR makes a hell of a lot more sense as a downsized camera format that supports high bit depth. Camera companies like the format, and are behind it. I think this means that sRAW will be a weird wrong turn that does not have a long market life.
</pure semi-informed speculation>
Peter
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