Most* PSD files contain a "preview" copy of the fully-flattened document (which is compressed.) Flat white image compresses far better, so that portion of the file doesn't take as much space.

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.