An Excel XLSX file is a zip file containing various XML files.
When doing a SaveAs in the DB Painter or in code, the XLSX file is created without any compression. This causes files to be much larger than they should be.
I saved a large resultset and the file was 30,440 KB. I opened it in Excel and did a Save As with a different name. The new file is only 5,760 KB.
When the SaveAs function creates the XLSX file, it needs to make sure it uses the compression method Deflate.
This is also a problem in PB 12.5.2 as well. See the discussion started by Brad Wery.