Adding large ranges of data, embedded images or complex formatting increases the size of an Excel file slightly. In some cases, however, Excel file sizes balloon out of control for seemingly no reason. Often, this problem is caused by Excel storing formatting settings for cells far outside the range of actual data. To fix the problem, delete the rows and columns past the end of your data. For further space savings, convert old Excel files to the XLSX format or the Binary Workbook format.
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Clear Unused Cells and Formatting
When formatting many cells -- such as changing the background color for an entire worksheet -- Excel usually only stores information about cells that contain data, even though it visually applies the change to all selected cells. In some cases, such as if you have an erroneous value in a distant column, Excel saves excessive formatting data and creates large files. To reduce the file size, select the column to the right of your last filled column, press "Ctrl-Shift-Right" to select the remaining columns, right-click in the selected area and choose "Delete." Select the excess rows below your data with "Ctrl-Shift-Down and repeat the steps used for columns..
Change File Types
Files created with versions of Excel prior to 2007, which use the XLS extension, can take up to more than twice the space as those with the newer XLSX extension. As long as you don't need backward compatibility with old versions of Excel, convert the file by performing a "Save as..." and choosing "Excel Workbook" in the drop-down menu next to "Save as type... ." You can shrink files even further and improve saving speed by using the Excel Binary Workbook file type, but this prevents alternative spreadsheet programs from loading the file.