Add --force-device-scale-factor=1.5 (or whatever fits your UI scale) as a command line option.
You could edit /usr/share/applications/spotify.desktop or wherever the .desktop file is located and change the line with Exec= to:
Exec=spotify --force-device-scale-factor=1.5 %U
Great tip! Here's what I did on 17.10 in case it helps anyone:
$ sudo gedit /var/lib/snapd/desktop/applications/spotify_spotify.desktop
Exec=env BAMF_DESKTOP_FILE_HINT=/var/lib/snapd/desktop/applications/spotify_spotify.desktop /snap/bin/spotify --force-device-scale-factor=2.0 %U