If you go to the page properties dialog, you have the option of importing a bitmap to use as the page background.

Obviously, your bitmap is then saved with the file.

Now, suppose that by mistake you used a very very very big file and you decided that this was not a good idea.

You go to the page properties again, and you say that you want no page background. Then you save the file. Unfortunately, the file size does not change a bit and the bitmap you used as your background is still saved inside the file.

Furthermore, for some reasons, once you have imported a bitmap, it is not listed among the available bitmaps in the "area->bitmaps" dialog, so you cannot delete it even from there.  Neither the bitmap file appears in the document navigator.

Editing the styles.xml file in a text editor to remove the corresponding bitmap entries works but is a bit of a burden.