[mockery]

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.

[oseph]

Interesting. I want to believe this is the case, but I feel like it is still a bug/quirk.

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!

[Dylan16807]

Is the preview the full resolution? That seems very overkill.

[mockery]

Perhaps, but it's quite convenient for any software that wants to ingest PSD files without the (nigh-intractable) challenge of reimplementing the entirety of Photoshop's parser and rendering systems!

[user-the-name]

The "preview" is actually the main image data from the version of the file format that was used before Photoshop supported layers.

It remains there, I guess, so that you can display the image even if you don't support loading and compositing layers.