Why are some 1080p videos so small?

I was wondering - the same video, why are some 1080p versions 10GB+ .mkv while others are 2GB .mp4? I can't tell any difference in video or sound, what's the real difference between the two files?

 

In one word: compression.

Depending on how compression is done and the source material you can have very little degradation for a good amount of compression, vicious knows more about that than me as do others obviously, but that's the core of it. Then there is also the fact that mp4 are 8-bit videos while you can have 10-bit mkvs (there are 8-bit mkv as well) too which means that it uses more space to store the same amount of information, 10-bit offers better quality at the expense of higher bitrates.

 

Matroska (.mkv) is a container format. The video streams within them can be of many different file formats. In addition each format can look very different depending on the profile/settings used when encoding from the source. MKVs using a 10-bit video stream, for instance, can look better than the 8-bit equivalent while being smaller. The trade-offs in this case would be a higher CPU consumption and lower compatibility. It mostly comes down to using the right format and profile for the given task and unfortunately you will see a lot of undesirable results since encoders choose to cater to different crowds (low-end hardware, poor players or internet connection) or the encoders simply aren't skilled enough (unsuited filter(s), format, bitrate etc.). That being said, the case has to be pretty extreme for there to be no noticeable difference between a 10GB+ and 2GB video if the source is a full length movie that hasn't already been heavily compressed.

 

Yes, both are container formats, but these days you very often see h264 being used for video, audio may vary a bit more, but you still see AAC used a lot. Now it also depends on the video due to the way compression works, some videos may be compressed more than others while not loosing too much quality: http://en.wikipedia.org/wiki/Data_compression#Video. You always loose quality when you compress, it's whether it's noticeable or not that matters. Then it also depends on whoever encoded it, the person who did the encoding might just have used default settings or not even bothered to strive for small size with higher compression and choose higher bitrates just not to have to worry about quality.

 

Also depends a lot on your display. CellPhone? 100MB would look fine. 120" projector? IMHO you would notice the difference.

 

I find it hard to believe a 10GB video looks the same as a 2GB version. I find it hard to tell between 4.37GB files and 5 or 6GB ones, but that's nowhere near as extreme.

Then again, it's entirely possible the movies were transcoded from a already heavily compressed source, and no matter how big the file gets, you can't bring back quality.