I can't download the backup of the world on my realm.

So first of all the problem I have is not there becuase of the connection of my phone to the internet. A half year ago, I could download the backups, but since then it isn't possible anymore. And a half year ago the world backups were already bigger than 426,7 MB but I could download them. If I want to download it now, i get the error message that a world whose file size exceeds 426,7 MB can't be downloaded. If I choose a backup which is smaller than 426,7 MB then I can download it.