FCP X exporting project to dual layer dvd

In FCP X 10.3.2, I am burning a project to a dual layer dvd. When I share to dvd, I select dual layer, burning to the internal dvd drive in my mid 2012 MacBook Pro. I notice the estimated file size displays 6.9 gb's. After the burn however, the file on the dvd is only about 3.2 gb's. The dvd plays fine without issue except for poor image quality, especially in darker scenes. I know this is only SD quality but I am wondering why the video gets compressed so much when there is plenty of room on the dual layer disc. I get the same quality and size when saving to the hard drive as an img file.

The source video by the way is Pro Res 422 and looks beautiful in the timeline.

Thanks all!

What is the duration of the timeline you're boring to dvd?
What format is it?

The timeline duration for this project is 1 hour and 57 minutes. The source is Pro Res 422 and was about 20gb's before importing into FCP X. The video was optimized after import to FCP X. The video codec reads mpeg-4 video in the FCP X info inspector.

That could fit on a single sided DVD easily. I'm curious, what is the source of the video? I'd have to see the original in QTPlayer or something.

I'm going to have to look at Compressor's default settings for DVD format encoding...