My source file is only 120ish mb, but my export file size is a whooping 17gb, and even after fiddling with some settings, at 720p, its still shows 3GB est file size.
Are you sharing as master or are you picking 720 or 1080? With those you can adjust the size in settings, but master is big because its not compressed.
Either buy compressor and customise the export settings or export a master file and use Handbreak or Quicktime to compress it to a more suitable size for your needs.