Credit to MadHatter

WINDOWS

Use your text editor of choice or if you don't have one use Windows Notepad
Opening with Notepad may or may not result in a fancy mess

1. Close Spotify
2. Open File Explorer and paste the following into the address bar %appdata%/Spotify
3. Open the file named prefs with your text editor
4. Change the numeric value following storage.size= . If storage.size= doesn't exist add it manually to the end of the file and assign numeric value. This value represents megabytes. One gigabyte equals 1024 megabytes. In the end, it should look something like this storage.size=1024
5. Save the file
6. Start Spotify
7. The cache should be reduced; not exceeding the size that you entered
8. To verify, check the size of the Data folder in the following directory %localappdata%/Spotify

Mac

Apple TextEdit.app works fine to open "prefs" or one can use some similar code editing program

1. Close Spotify
2. Open Finder and press ⇧ + ⌘ + G or Open Spotlight (⌘ + Spacebar) and paste the following into the address bar~/Library/Application Support/Spotify
3. Open the file named prefs with your text editor
4. Change the numeric value following storage.size= . If storage.size= doesn't exist add it manually to the end of the file and assign numeric value. This value represents megabytes. One gigabyte equals 1024 megabytes. In the end, it should look something like this storage.size=1024
5. Save the file (⌘ + S)
6. Start Spotify
7. The cache should be reduced; not exceeding the size that you entered.
8. To verify, check the size of the Data folder in the following directory ~/Library/Caches/com.spotify.client/