There's a checkbox in the packaging settings in Unreal Engine's Project Settings down a bit to select Compressed package files, have you trying engaging that already (if you are on UE)?
Viewing post in ClockWorld jam comments
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.
That doesn't quite sense to me, unless I really don't understand the concepts here. Unreal didn't cut anything from your game though...? Meaning, I downloaded your 700MB or so zip, but when i unzipped it, it ballooned to 6GB on my hard drive. So... it was there already compressed in the zip file, no? Or do you mean somehow Unreal created the extra 5.3GB there on the spot?!