Hi René, 🙂
The YouTube video should recalculate its size. It seems something is preventing this. My best guess is that your issue might probably caused by some conflicting third party plugin or some extra code snippet as we don’t encounter this issue with our demos.
Could you deactivate all plugins that have not been installed with your theme during its setup and all existing custom PHP code snippets please? And then, could you confirm me that the issue is still persisting please?
Please also make sure that you have the latest theme version and that all recommended plugins are up-to-date. Finally, you could also consider clearing all activated caching system as well as your browser cache before investigating any further. If you want to read more on what browser cache is and how to clear it, here is an article that might help you: How to Clear Internet Cache in Every Major Browser.