Word file size reduces to half despite increases in text

[LMatth]

I have been writing a nonfiction book using MS Word 2007, alternating between
a Windows XP, Pentium D PC w/1 GB RAM, and a Windows Vista, quad core PC w/3
GB RAM. The book is currently about 750 manuscript pages. When I last saved
it after working on it, the size was 1,922 kb. I always save it as a .DOC
file (saved for Word 97-2003 rather than a .DOCX file saved for Word 2007).
The file is backed up on multiple hard disks and flash drives. However, the
last time I saved an updated copy in the exact same manner as I always do,
the file size showed as 945 kb, rather than the earlier size of 1,922 kb +
additions, as I expected. This immediately raised my alarms, so I saved it as
a separate file with a different name in the other back-up locations to avoid
overwriting the earlier files with the larger file size. I examined the
smaller file and could see no obvious reason why it was almost half the size
of its predecessors. All pages were present, and the formatting appeared
normal. To make matters even more bizarre, when I saved the new 945 kb
version under the new name in one of my back-up locations, the same file that
showed as 945 kb in the original directory saved at 1,887 kb in the back-up
directory (I note that because I was transferring it from one physical
location to an off-site location, this particular back-up location was on the
same 512 MB flash drive--with 70.7 MB free--as the source file). Also, when I
saved the 945 kb version as a .PDF file, it grew in size compared to its .PDF
predecessor (from 2,336 kb to 2,433 kb). Can you explain any of this and/or
suggest what I might do about it?

 

[grammatim]

Did you accidentally save it as a Word2007 file (.docx), which will be
a lot smaller than a .doc file? Like, if you gave it a new name, maybe
you forgot to change the save-type?

 

[LMatth]

No. As was mentioned in the question, I saved it as a .DOC file (Word
97-2003), and not as a .DOCX (Word 2007) file. The extension displays as .DOC
as well.

 

[grammatim]

You might have _thought_ you did, but it takes an extra step that you
might have forgotten to do while operating on autopilot.