There's three things you can try out.
1. Go to 'your Libary' on Spotify -> press the optionwheel in the upper right corner -> Select 'Normalize volume' option off and see if that works. If not, try to restart the Spotify app and try again. If it dosnt change anything, try to set the 'Normalize volume' back to on.
2. Go to the same direction as above -> just now try to scroll down to until you see 'Equlizer'. Press that and see if some of this settings are changed on the phone (Equlizer are effects that normally are used on Android phone itself).
3. See if the media volume on the phone is lowered by going thru the options on the Android phone.