the high freqs on youtube are less defined and clear than on spotify so the sound on youtube is in other words... warmer/smoother. Since its smoother, you probably can turn up the volume a bit more resulting in a wider stage, simple as that.

thats my observation when i compared the same songs between spotify premium and * regular* youtube music videos of them

i do use youtube to listen to music sometimes when i want to relax late at night (: even if i can have higher quality on spotify.