When I use Spotify connect or select an available device (Samsung ms-650 soundbar) to cast to within the Spotify app, the volume control using the phone volume rocker, jumps in what seems to be about 6db (I.e. Twice the volume!) increments and nothing is available in between. Once the Spotify volume UI appears (after the volume jump!) the UI volume slider has no effect on the volume.
Spotify is always looking to improve the app, and we appreciate all the feedback we get from users like yourself.
However, since there are lots of possible improvements that can be made, we'd like to get an idea of which ideas are the most popular. As a result, we have a dedicated Idea Exchange board where you can create ideas about the things that matter to you.
We suggest you create an idea for your suggestion by going here and getting others to vote for it. The more people that vote, the more likely it is that Spotify might implement it in the future!
Hope that helps - let us know if there's anything else we can do for you 😉