Thank you for contacting Aspose support.
First of all, please note that there is a vast difference in internal structures of XLS & XLSX/XLSM file formats. XLS is a binary format which tends to occupy more disc space whereas the XLSX & XLSM are Open Office XML formats therefore the file sizes of XLS & XLSX/XLSM are not comparable.
It is quite possible that a XLS of size 2MB can be saved in a few KB of XLSX/XLSM because the later formats occupy lesser disc space due to their compressed format. You can simply change the extension from XLSX/XLSM to ZIP and inspect the file contents in any Zip utility such as WinZip or WinRAR, you will notice both XLSX & XLSM comprises of a bunch of XML files.
Hope this helps a bit.