Reduce size of the Xcode application
Although it's not directly related to the Xcode app size. There are multiple places where you can delete files to free up some space.

1. Derived Data
Xcode keeps data about your projects which includes index, build output and logs. Go to ~/Library/Developer/Xcode/DerivedData/ and delete the folders for projects that you no longer need to keep this data around for.

2. iOS Device Support
Xcode is storing information about the devices you have used for development. Delete the data no longer needed in ~/Library/Developer/Xcode/iOS DeviceSupport/.

3. Archives
Delete the one’s you don’t need in the Organizer, or go to ~/Library/Developer/Xcode/Archives/ to delete them in bulk. Be careful not to delete archives for which you still need the dSYM data for debugging.

4. Simulators
Simulators are stored under ~/Library/Developer/CoreSimulator/Devices. For each version of iOS you are building for.

5. Documentation Browse and delete unnecessary data under ~/Library/Developer/Shared/Documentation/DocSets.