It's because the 1Ds is really quite old, and Canon didn't embed any larger preview in the RAW file like with later cameras. So the RAW file contains just the small 100pixels or so preview of the photo, while for example 1DsII RAW contains a much bigger, about 3 megapixels, preview.
If you are on the Mac, you could turn on OS X's RAW rendering, either in the preferences of PM, or just temporarily using Q key in the preview window. But it will get slower because now the system has to render the complete RAW file instead of just viewing the embeded preview.
