As many folk have moved to many megapixel images you see the document sizes leap in size. BUT THAT'S NOT YOUR ISSUE.
It appears you are seeing a limitation in this app. To conserve disk space I suggest you save that in a compressed folder. XP supports that so here's a link.
I have a 1-page document with some text and 6 jpg files with an average size of 200Kb. The text and pictures are slightly outside the margins established in the Master Page. When the document is saved, the size is about 35 Mb. If I then bring the text and pictures inside the page margins, the document saves at about 1.5 Mb. Why is the file size so large in the first save??
This is Publisher 2007 with Windows XP.