I have some xls files without around 12 tabs containing data, links to other excel files, some data validation, and some conditional formatting. Saved as xls, they use around 1.2-1.5 mb each, but if I saved them as xlsx, the file size is reduced to around 1/3 of that.
What is it about xlsx file format that makes the files smaller; is it something to do with the links to closed workbooks? When I tested saving an xlsx file as xls that was nothing but links to closed workbooks, it gives this Loss In Fidelity message:
"Some formulas in this workbook are linked to other workbooks that are closed. When these formulas are recalculated in earlier versions of Excel without opening the linked workbooks, characters beyond the 255-character limit cannot be returned."
And the xls version was about 4x larger than the xlsx version.