Good point onedtent, but it probably won't have much effect on the final file size of the document. The allow fast saves has to do with the order in which the data within the document is saved.
From what I understand of this function, when you turn on allow fast saves, this just makes it so that any added data to the document is saved at the end of the binary file instead of where the actual edits within the document were made.
This has a tendency to make the document less 'stable' and more likely to corrupt, especially if the creator of the document hasn't used Styles for formatting the document and has applied direct formatting throughout the document.
When you turn off allow fast saves, edits to the document aren't appended to the end of the binary file. Instead the document is saved from beginning to end in one-fell-swoop (as we say in the mountains). This makes for added document stability.
Allow fast saves makes it so that if you edited paragraph 3 out of 100 paragraphs total and also formatted that edit without using styles, those edits, including the additional binary language needed for the formatting, would be placed at the end of the document. When you opened, or printed the document, the computer would be reading paragraph 1 and 2 and paragraph 3 up to the edits and then would have to skip to the end of the binary file to read the edits, then once done, on with reading paragraph 4 and so on.
If you turn off allow fast saves, the computer and program (Word in this instance) reads the binary file from beginning to end. Of course if direct formatting is applied, the computer and program have to read all that additional information, whereas if you setup the formatting using Styles, then the computer and program only have to read the formatting information once, then pull that information from memory to apply throughout the document without having to read all that individually applied formatting information.
So, ultimately, turning off fast saves AND using Styles for formatting will cause the binary file size to be somewhat less, but won't really matter concerning the graphics contained within the image.
Too much information? Probably so! lol
dodomfcg