From what I understand, the smaller something is before you zip it the less the file size will reduce by.
For example, our project was 6GB. But only a small part of our project was essential, so Unreal could cut out a lot more to create the zip. In your case, your project might only be 1GB, but more of it is essential for rebuilding your project.
Not sure if that made sense... Anyways here's a link on how zipping works.