I think Spotify should drop the pop-up for something like a bar at the bottom you can change yourself, or a bar at the top to just show you the volume level as you change it. Many apps across iOS have done this and it really improves the experience.
Updated on 2019-01-08
Your idea was submitted a while ago and unfortunately didn't receive the amount of kudos needed to stay active in the ideas board (100 Kudos per year).
In order to keep the Ideas Exchange clearer I'll close this idea for now, but this doesn't mean Spotify rejected this idea.
If you think this is an idea which needs to be implemented, you're welcome to post it in slightly different again :)
Have a nice day!