I mean "File -> Export..." then choosing Microsoft Office 2003 XML. Actually I wanted XLS file, but didn't put an extension manually, so LibreCalc exported to pure XML.
I used LibreOffice 7.4.7.2 on LMDE 6 (pretty old, I think).
Now I tested it with the same file on LibreOffice 25.2.4.3 on FreeBSD 14.3. Results are the same: 439 Mb file.