When you have layers in a document, and you've selected Maximize Compatibility in the file handling preferences, when the document is saved Photoshop stores a flattened composite along with the layer information (essentially, one more layer).
If the document is just a Background to start with, it does not create a separate flattened composite, since that's what the Background already is.
If you never have to worry about whether you want someone to be able to open your document in an older version of Photoshop, or another application that reads PSD files, you can consider setting the Maximize Compatibility setting to Never, in which case Photoshop will not generate the flattened composite and the save will proceed faster and produce a smaller file.
Thanks Noel. Ok, I'm going with never. I thought I had that set to never. What do you set it to?
I set Maximize Compatibility to Never myself. That setting suits me in my work environment.