You want to see something "fun" check out Apple's Numbers XML format from before they switched to protobuf. A simple 1,2,3 resulted in over 1 megabyte of XML.

At a previous job I'd been tasked with developing support for importing Numbers files along side our existing Excel and CSV support. After a couple days we rightly gave up as the tiny fraction of people who actually wanted to import Numbers files was outweighed by it's massive complexity.

We ended up just adding instructions for Numbers users to export to CSV