Hi all!

I have tried making movie-only MKV's of a couple of my Blu-rays (Speed Racer & The Good The Bad and the Ugly). I have followed the guide found here:
http://adubvideo.net/how-to/backing-up-blu-rays-with-bd-rebuilder

In settings, I select to use a custom size... 18Gb for GBU, 13Gb for Speed Racer.
The process seems to work fine... no errors or anything. However, the resulting files are at least half of the size identified in the Custom Size settings (7.5Gb for Speed Racer and 8.7Gb for GBU).

Can someone explain this to me? I feel like it is some setting I have missed somewhere, but there aren't that many settings to play with in BD Rebuilder. I am of the understanding that the default settings for the additional apps (ffdshow, matroska splitter, etc) are fine.

Cheers!
- Ryan.