Eugene Meidinger notes that Power BI Desktop files are a bit smaller now:
I was working on a demo for my upcoming Pluralsight course, and I noticed something odd. It used to be that a empty PBIX file was 123 KB, but some point since May 2017, the file size has become 10 (!) KB. So what’s the cause of the difference?
If you rename a .pbix file to .zip, you can crack it open. If we look at two nearly empty files side by side, we can see the difference comes from the data model. In this example, each data model has a single value that I manually entered.
Yet another reason why it pays to keep up to date on Power BI versions.