The thing with music today is that producers want the songs/albums to be as loud as possible, but there's a physical point where when you master a recording too loud it distorts.
The probably best example is the Metallica Death Magnetic album:
So louder is not always better, the louder you make it the more information and dynamics you loose. This has nothing to do with your volume level in your computer, this is all about the volume inside the file.
If you ever recorded for example a live show with good equipment, where you can set your own recording levels, nothing with auto gain like all the digital cameras have for example, you'll notice how insanely loud studio music is compared to that.
There also seem to be small differences between the two versions. For example the end of the song, in the EP version the echo of the very last "time" while it fade out is a lot longer.
I think the vocals in the entire song sound slightly different, maybe it's just the mastering but for example at 2:16 the vocals sound way different, like an additional filter was applied to make it sound more lo-fi.