Playground storage is not expandable. To make sure you always have room for new releases, the least played games are automatically offloaded to free up space.
Just tap any offloaded game to redownload it; if you're on Wi-Fi, this happens in the background.
Playing offline?
Download the games you want ahead of time, since offloaded games need Wi-Fi to re-download.
Favorites are always safe.
Games on your Favorites list are never offloaded, no matter how often you play them.