Originally Posted by goldscraps
It really depends on what is in the PSD file. Some of what causes a PSD file to be larger is that it carries a preview within the file also. On some of my larger template files, I put a blank layer at the top to block the creation of the preview data so the file is about 25% smaller.
As with the TIFF, it also depends on what is in it. If there are more large intricate shapes with transparent areas around then (like doodles or something), then there will not be as much compression available. Typically, the compression will make the TIFF a little smaller than the PSD.
It also depends on how the TIF files are saved - if you don't compress and select specific options, the TIF files will be larger than the PSD files. I have an action that I run to be sure the correct options are set so that the TIF files are smaller, but usually it's not by very much in comparison to the PSD files. And personally, even though TIF is recognized and compatible with Photoshop, I still prefer the native PSD files