It's essentially a serialization of the binary format to XML.
ODF 1.4 is around 1,100 pages across all 4 parts whereas OOXML is over 6,000.
[1] https://stephesblog.blogs.com/my_weblog/2007/08/microsofts-f...
[2] https://ooxmlisdefectivebydesign.blogspot.com/2007/08/micros...
[3] https://www.robweir.com/blog/2007/01/how-to-hire-guillaume-p...