Hi,
You need to do one thing —actually two 🙂 Find out what caused the size and delete it.
To do this right-click or control-click on your library and choose Show Package Contents. This way your library will open as a folder in your hard drive. Choose to show as a list and click the Size column so you can see everything from big to small. It’s probably render files or if you have transcoded media like optimized and proxy, it could be that. To save space, you can freely delete the Render Files folder And Transcoded Media folders. Also, there is an Original Media folder. Be careful about that, if you don’t have a copy of those files inside the Original Media folder somewhere else on your hard drive, don’t delete them.