I'm not sure if it will help you, but it definitely helped me. I found
a program that shrinks Excel file sizes. The reasoning they gave was
that for some reason while creating a sheet, it redundantly saves
formulas and data that you previously deleted or something. Anyhow, I
went from a 10mb file to a 3mb file. Then i shrunk it again to see if
it would shrink again. It went to 600k. I couldn't shrink it anymore
then that. I don't quite remember the name of the program, but if you
google "Excel file shrink" or something like that, you're bound to
find it or something similar.

One option in versions prior to 2007 is to select your pivot table and look
at the options.  I can't remember the exact name, but uncheck the default
option to save the data with the Pivot table.  The idea being that you don't
want to keep a large amoutn of duplicate data.  However, select "refresh on
open."
I can't find that exact option in Excel 2007.  I believe you unselect the
option "save source data with file."   I have a hard time with Pivot tables
now.  My productivity has gone down the toilet since using Excel 2007.