Most tiles have a fixed space for icons. I’ve marked it with a red outline in the following screenshot:
You could use a Super Tile if you wanted more control over the placement and size of the icon.
From a quick check of the Spotify Web API documentation, it looks like there is an option to set the player volume. You would need the requisite HTML + JavaScript skills to modify the custom tile if you wanted to implement it though.
If your player device is integrated with your smart home, you may be able to control it with the native music player tile. And there’s a Vertical Range Input (Slider/Dimmer) custom tile that can be used for dedicate volume control for player ‘things’.