I have just converted an NTFS partition to ext4, however the total space seems reduced from 421G to 415G. Where did the 6G go? And, the reserved space is grown to 199M in ext4, much larger compared to 78M in NTFS, why?

The partition is mainly used for movies/musics, so most files are very large (>10M each). I want to use ext4 file system, is there any suggestion?

mkfs.ntfs:
    /dev/sdb4             421G   78M  421G   1% /mnt/mmedia

mkfs.ext4:
    /dev/sdb4             415G  199M  393G   1% /mnt/mmedia

It's also weired that the remaining size of ext4 is 393G, shouldn't it be 415G or 414G? What happened to the disappeared 22G? Compared to NTFS, ext4 seems eaten 28G in total.