Since the release of Excel 2007 it has been storing data as an XML file. This is a good practice if we want to exchange spreadsheets between different platforms, or even operating systems, but when it comes to complex calculations, which contain hundreds of thousands of rows, such documents can take up a lot of disk space. This can be overcome by saving with the use of the XSLB extension. You can read about the advantages and disadvantages of this approach in the text that follows.
A document with an XLSB extension is a binary Excel file, which means that data is stored in binary form instead of XML. These documents support all standard Excel content, including macros. Reading and writing from such files is much faster, and they take up less disk space.
You can open XLSB documents with any MS Office package, as well as with most open source packages for similar purposes. Conversion from XLSX to XLSB format is done by saving the document using a different extension.
If you pay attention in File Explorer you will see that the XLSB file takes up much less space. In this example it is somewhere around 30-35%.
What is the potential disadvantage of working with such files? As mentioned, XLSB files can have macros without being able to recognize it based on the extension. This means that they may potentially contain harmful contents. As computer viruses, malware and ransomware content spread more and more via e-mail, if you receive a document like this from an unknown sender, never open it!