Compression
Safe. Always on.
Compresses the file at the tightest standard setting.
Why it takes space
An .xlsx workbook is a ZIP file holding XML parts: one per worksheet, plus the text values, the styles and the rest. Whatever saved the file chose how hard to compress each part, and many programs choose speed over size.
Worksheet XML repeats the same tags on every row and every cell, so it compresses very well when the compressor is given the time to find the repeats.
What ShrinkExcel does
ShrinkExcel compresses every part again with Deflate at level 9, the tightest setting of the method every .xlsx reader understands. When a part was already packed smaller than that, it keeps the part as it was.
Nothing inside a part changes. With every reduction switched off, each part comes out byte for byte the same once unpacked, and the check before download compares them.
There is no setting for how hard it tries. A slower compressor, Zopfli, gained 8 to 14 percent more at about 67 seconds per megabyte, which would stall a browser tab for minutes on an ordinary workbook. The fastest Deflate level saved about a seventh as much.
What you will notice
Nothing. Excel and every other program unpack the file the same way, and every value, formula, format and object is where it was.
On most workbooks this is where most of the saving comes from.
Doing it in Excel
Excel has no setting for how tightly it packs an .xlsx. Saving as an Excel Binary Workbook (.xlsb) often gives a smaller file, but that is a different format, and this tool does not read .xlsb.
It runs on every workbook you drop on the front page.