Yes, a Background cannot support pixels beyond the visible edges. Therefore, if you want to preserve those pixels, Photoshop must make your background into a full layer.
Yes, if you save a document with pixels being maintained beyond the edges on one or more layer(s) it will be larger than if you deleted the pixels.
A TIFF can support saving data beyond the visible edges of layers, but a JPEG cannot.
Thank you, Noel. I wasn't too clear in how that worked.
I think that, for the most part, I will keep the "cropped" pixels during the editing session to allow me to change my mind but, when ready to save, I will discard those pixels by changing the Layer 0 to Background again (unless there is a good reason not to for a particular image.) It helps a lot to understand the mechanics of it, so I really appreciate the thorough explanation.
Since I save in Tiff, ProColor, 16 bit, my files are big enough as it is. Still, it is good to have the option.
Life was simpler when cropped pixels were just *gone*, but now we have this ability, and it does actually seem to be getting more and more polished. I believe I've noticed at least some of the filters actually will work on the pixels you can't see now, so that if you do end up anti-cropping (or moving or whatever) the image you won't be left with a weird edge.
Out of curiosity, why not save in the PSD format?
Fear and convenience. I fear that, at some point, PSD will become obsolete, whereas tif is less likely to since it is recognized outside of Adobe. I try to be forward-looking, which is why I work in ProPhoto and 16 bit which, at this point, is mostly an inconvenience.
Convenience because the printer that does our giclees uses tiff so it's easier to save my master copies that way). I flatten his copy and keep mine layered. That practice extended to my personal photos, even though I have to change to 8-bit and jpg and sRGB for those I wish to print in a less professional setting.
Another convenience is that, without having to open Bridge, Windows will preview tiff files and not preview PSD.
I do have to use PSB (or whatever large the format version of PSD is called) for my really huge files which, by the way, I notice are opening a bit faster in CS6.