I created an excel sheet with python xlsxwriter. When i resaved it with libreoffice calc the size decreased by more than 60 percent. why is it ? Is there any way i can mimic this in python to reduce my file size?
As far as I know that also happens with files produced by Excel so it isn’t an issue with XlsxWriter files. You can verify this by saving the file in Excel and then LibreOffice to test the change. 
XlsxWriter files should be the same size (with small percentage differences) as Excel files since the XML part is almost exactly the same. The zip compression may differ though.
Excel sheet size reduced on resaving