Take a PSD that has many layers. Look at its filesize (mine is ~70mb). Add one layer to the PSD, fill it with white, and make it the topmost layer. Save it as a new PSD and compare the filesizes.
The new PSD with the white layer is 55mb. Why?
Depending on what your layers look like (how many, how much they cover, etc.) it's not too surprising that the preview image could take a substantial fraction of the total file size (sounds like ~20% in this case!)
* I believe this behavior can be toggled off with an option.
It remains there, I guess, so that you can display the image even if you don't support loading and compositing layers.
In Photoshop's preferences there is an option under File Handling to disable image previews, and toggling this seems to have no effect on what I described above. Strange!