Burning a project to Blue Ray Disk using FCPX

I finished a 35 min HD project using FCPX and now I want to burn a Blue Ray Disk when I check the summary and the size of the disk is only 8gigs how do I get it over 20gigs using a 25gig blank Blue Ray Disk or is this the size Im gonna get for the disk size and 35 minute length I just thought It would be more than 8gigs

The size will depend on a lot of things, namely the project size and settings you used to encode for DVD. 8 Gigs is well larger than standard DVD, so it's probably correct based on your project and encoding. Just burn it and see how it looks.

Consider that at that duration and file size you'd be getting about 90 minutes, a feature film, on a Blu-ray disk. Your size sounds about right if not a little on the large size.

Keep in mind that you can only push the data rate so high before it hits a wall and as you get closer to that wall there's usually diminishing returns on the quality improvement.

You'd be targeting an ideal bit rate, not file size..

To use a loose analogy, imagine a professional baseball player who at 225lbs is a great power hitter. Would your objective be to push him to 450lbs, at which put he'd have lots of fat, be overweight and the power would be replaced with a reduction in the ability to maneuver. More is not not better. More can be too fact. More can mean a data rate too high for some disc players to handle.

Your aim is not to fill the disk but find the optimal data rate for the content.