file size indicates how compressed the movie is..

a HD movie can be compressed to a small as 1GB from a 40-50GB Bluray disc.

a 720p rip will have smaller file size than a 1080p rip

most HD movie compressed to mkv these days. some m2ts, ts etc...again, the type will affect file size.

the audio codec also determines the file size. AAC, AC-3 is normal DTS audio...

movie with HD audio like HD-MA or TrueHD will make file size larger.

general rule of thumb, the bigger the file size, the better the quality (both audio & video)...avoid anything smaller than 4GB...above 10GB is good, above 20GB is better...uncompressed is of course, the best.