When I export a 15 minute video at 1080, 30 fps, medium quality VIRB Edit (3.6.0 on Windows 10) says it will take about 660Mb. After export the video is 1.7Gb. Why? It has wind noise at 65% volume, video, and three title text overlays for 5 seconds each.
The file size estimate is just that, an estimate. It is very hard to make accurate predictions, usually we're pretty close but in some cases (like yours) we're unfortunately quite off.
We can see if we can tweak the algorithm again, but it's not at the top of our agenda.
I apologize for not having a more satisfying answer.
