If you are converting the Blu ray videos to .mkv, then there is no difference in the burning process, as you will be burning as data files. If you are burning blu ray structure to a disk, you'll need to use UDF 2.5 file system. Burning with ImgBurn, it will normally warn you if you are using the wrong file system and ask to correct this for you.
You'll find that approximately 2/3rds of all commercially made Blu ray movies are released on 50gb disks, and many will not fit onto a 25gb disk without compression even if you are only copying the main movie.
If you need to apply compression when making an mkv file, look at programs like Ripbot264, BluRip or MakeMKV.
What program do you use for streaming?