Is it right that the game is ca 20 GB big ?
The default app is under 4GB in size. When you install additional scenery regions the app size obviously increases because the aerial images take up quite a bit of storage.
You have bought Aerofly FS 2021 the first day ?
In this case, you may have downloaded regions. The downloads are from Apple. But IPACS changed the way to download regions, they use their own servers, so you redownloaded the regions from IPACS. But Apple downloads remained. So the storage reaches 20 Gb instead of 12 Gb with all downloads.
Unfortunately, you have to completely delete Aerofly FS 2021 to delete dead downloads too. And redownload the application and the downloads.
Unfortunately there’s no region manager to delete specific region