I believe Symbols.zip contains debugging symbols used for generating stack traces when an error occurs. It should be safe to remove this, though you might have difficulty diagnosing crash reports.
As an aside, if you're getting 1GB of symbols in a non-debug build, it sounds like something is going wrong with the build system, so this might be something worth reporting to Unity using the bug reporter.
Oh, also check the player settings and make sure you don't have "full" selected for any of the Stack Trace options, as this generally greatly increases the build size. "ScriptOnly" should be fine.