The storage used by an app is not displayed accurately on the iOS system.
Duplicated data is stored only once. This means that if you use a 10GB clip from the Photos app in your project, you will see that both the Photos app and Alight Motion are taking up 10GB of storage. However, if you delete the clip from Alight Motion, the free space will not change, as the video is still stored in the Photos app.
Therefore, although it might look like Alight Motion uses a lot of space, it is shared with the Photos app or the music library.
If you'd like to free up space in your device, you’d need to delete media files from both Alight Motion and the Photos app/music library.