I tried dissecting the .xls (2003 compatibility mode) file, and it
appears that Excel 2007 adds a reference to something called
themeManager.xml when you save the files in the 2003 compatibility
mode (i.e. .xls). I couldn't find any useful information about
themeManager on Microsoft's website, but from what I understand, it
defines the formatting of a document. This additional xml metadata
causes the the file to bloat. And since native 2007 (.xlsx) is a
zipped file format the file size increase is not noticeable, but when
saving to .xls format (non-zipped) the file size increase is
striking.
I posted this question on GetSatisfaction.com and Kathy Jacobs
responded as follows:
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It will happen with most files. The new file type is a much more
compact set of files that are then zipped together. When you back
save, the graphics and features for both versions are put together in
the old format.
One way to think about it is to think back to the changes when Office
97 came out. The compression rate for the new file type is another
level better - but for the old programs to read it, both sets of
information have to be there.
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(see:
http://getsatisfaction.com/microsoft...ile_size_by_5x)
So the only solution is is to remove all formatting and theme before
saving the to .xls file format (compatibility mode) using Office 2007