Why does LO save MS office created files with a larger file size?

Why does libre office save MS office created files with a bloated file size ?
I opened a 950kb docx MS word file that was emailed to me into Libre Office.
After a small edit I saved it in the same format to the alarming file size of 3.5mb.
I saved it in odf format and the file size remained similar at 958kb.
It happens even if I just open it and save it again without any edits.
The same happens with excel files.
This is annoying whilst sending files back and forth via email.
Is there a setting somewhere that can avoid this ?
Thanks in advance.

What version of LibreOffice and what version of Microsoft Office?

this is still quite a bad issue, was sent a 2.2MB .docx file this morn not sure what version of MSoffice but adding a few characters (8=my name and a date, again 8 chars) the file was then saved resulting in a 6.5MB file…

File size is dependent on many factors relating to the content of the file. What does the DOCX contain? If you are seeing a significant increase in size it may be due to something like EPS/WMF/EMF vector art being converted to raster (e.g., PNG) equivalents. The process of internal conversion (DOCX → ODF) when opening a file will change certain objects whose formats are not natively handled. Saving to ODF will then sometimes re-convert to a more ODF-supported format, resulting in a corresponding file size. OOXML and ODF are quite different.

Don’t know if this helps, but if your coworkers are using latest Microsoft Office 2013, then you can ask them to save files in ODF file format (or set this format for default).



More info: http://www.zdnet.com/microsoft-office-gets-support-for-new-odf-and-open-xml-file-formats-7000002607/



And differences between MS-Office and ODF format: https://office.microsoft.com/en-001/excel-help/differences-between-the-opendocument-spreadsheet-ods-format-and-the-excel-xlsx-format-HA010355787.aspx

Office, no matter the version, can’t write ODF properly.