Why keep larger file size if quality is the same?

[ShiinFan]

I've recently been finding new torrents of the same movies I already have that seem to be then same resolution but the file sizes are sometimes twice as large. For instance I might have XYZmovie.mkv at 1920x1080 resolution that might weigh 1.9 gigs but the ABCmovie.m4v version of the same movie with the same resolution might weigh 5.5 gigs.

Should I keep the larger file size version if I can't tell the quality is the same? The same goes for iso files. I have iso files that can weigh over 20 gigs but the quality seems to be the same as the individual movies that weigh 2 gigs. Why are some iso files so much larger?

Maybe I would see a difference on a larger TV screen or something.

 

[Casshern2]

This has to do with how they were encoded I would say. Different bitrates produce different sizes. The higher the bitrate (potentially higher quality) will be heavier than lower bitrates (potentially lower quality).

 

[Porni]

Why are some iso files so much larger?

Because video on disks is uncompressed. Sometimes the video cannot be compressed and the size may be larger than the original, depending on the dynamics and detail of the image.

 

[SamKook]

It's hard to tell sometimes because a lot of people don't really know how encoding works so sometimes you'll have bigger files that look worse than smaller ones and other times the bigger one will look much better or both might looks pretty much the same.
The same video encoded with different speed settings will look pretty much identical but will have varying file size.

Since I was curious, I tried every presets on the same video to see what the file size would be and here's the result:
Original video: 2.1GB
ultrafast: 1.97GB
superfast: 1.10GB
veryfast: 845MB(that one is really odd since it took about as much time to encode as the veryslow preset. Not sure why)
faster: 951MB
fast: 979MB
medium: 947MB
slow: 927MB
slower: 888MB
veryslow: 818MB
placebo: 839MB

For wmv files, those are usually streaming website rips and people just re-encode them at the same bitrate and codec as the original which is kind of a waste of space since you lose quality anyway by re-encoding and you could save a lot of space by not using a specific bitrate. The streaming website have to or it would cause issues with a variable bitrate.

For bluray iso files, they're this big because there's plenty of room on the disc so they compress it as little as they need to(it's not uncompressed since even a lossless compression would be close to 300GB and uncompressed would be much bigger but it's not heavy on the compression like with movies you find on websites) so it looks better. It might not be that noticeable but they do look better than any other source.


As for what you should do, people are sensitive to different things(different codecs/filters will cause different issues in the video and some you might not notice or care about and others might stick out like crazy) and if you don't notice the difference, there's not much reason to keep the bigger one.
The weakest spots when encoding is where there's movement, dark scenes, diagonal lines and fine patterns(that I can think of atm). Try to find a few spots with those and if you still don't see a difference, don't bother with the bigger ones.

If you plan on getting a 4k TV in the near future, that might make stand out some issues you may not have noticed before since the upscaling can exaggerate some issues but other than that, shouldn't matter much.