I tried an experiment to see what would happen. Here is a screen shot of the folder where I stored the test photos
The photo was taken with the Canon 20D in RAW. I Saved it out of the RAW converter from CS3 as a tif file. The image has the name img_5421.tif. I then loaded it into CS3 and saved it out as a PSD file. I then loaded it into CS2 and saved it out as a PSD file. The image was in 16bits. You'll notice that the CS2 file came out a lot smaller than the CS3 file??? Also note the ones marked test5421 were the same test, but with a 8 bit file.
I don't feel like Adobe would have added a undisclosed compression to the PSD file format in CS3. If they did, then those files would not be backward compatible and that would cause a major snafu in the industry (if it was undisclosed). But then how does one account for the file differences I see in my test???
Could the RAW converter be the difference, I wonder?