Haiku is versioning your design files in Git.
In the current implementation, every time you make a design change in Sketch an entire copy of the Sketch file is maintained, enabling you to go back in time to any revision of that file.
The problem here is the disk usage, of course — every change stacks up, especially with large design files.
We have plans to automatically offer 'LFS', which is an extension to Git that manages large files much more efficiently.
If this is causing pressing pain in the meantime, there's a way to remove the sketch files from your projects' histories, but it would require some command-line work. Essentially, follow this process here: https://help.github.com/articles/removing-sensitive-data-from-a-repository/#using-filter-branch to target your individual .sketch (or .ai) files, and then run