Symptoms: Users report slow upload or download of Excel Files, or report that SharePoint refuses to upload an Excel file, prompting the user to save the file locally instead.
Cause: Formatting or adding formulas to whole rows or columns causes Excel to recalculate or reformat thousands of empty cells on each save and makes the size of the Excel file grow exponentially. While technically not a SharePoint issue, users first notice the disproportionate file growth when the workbook size reaches a point that causes performance issues when communicating with the SharePoint server or when the default SharePoint upload limit of 50MB is exceeded.
Solution: Clear unneeded formatting and calculations from the workbook’s empty rows, as described in this post http://www.spreadsheetshoppe.com/2/post/2012/01/how-to-reduce-excel-file-size.html. Users may also wish to save the file as an ODR file, which will remove the blank cells. The user can then save the file back as an xlsx with the same file name. In one instance, the size of the workbook was reduced from 52MB to 235K.