Hum, the sizes you posted looks suspiciouly like.. oh wait these *are* the dimensions of the embedded JPEG previews written by the cameras!
Joke aside, the actual preview image size is camera model dependant indeed. The codec pack ships with a control panel applet that let users decide if they want to always use the JPEG preview, or decode the raw data.
Our Canon codec ships with the "always JPEG" option checked, but you can turn it off if you want. The drawbacks will be less speed (the codec does not become slow
, compared to other raw converters, but it will not be as fast as when it just extracts and unpack the JPEG data...). Also, the raw conversion may not look as vibrant as the JPEG, but you'll get the full images.
For most purposes (thumbnails and quick preview in Explorer or Windows Photo Viewer) the default options are fine.