www.makemkv.com

I had ripped a Blu Ray disk to Blu ray format using another ripper before I discovered MakeMKV. The resultant m2ts file was about 27 Gbytes. I then asked MakeMKV to convert that file to mkv format (I pointed makemkv to the stream.index file). It read and processed the file but the rsultant file was only about 17 Gigabytes. I thought there would be no transcoding. Can someone comment on wether this process is correct or do I have to re-rip the original disk using MKV.

Any advice appreciated

Thats because you didn't rip the main movie (17GB) you ripped the entire disc which was 27GB in total (all the extras on the disc).