YouTube Video changes size into small after page loads fully

Hi!

Thank you for the great theme and your work! Iam really happy with my new theme despite one feature: if I embed a YouTube Video (which in the editor is displayed correctly) it changes its size to very small, when the post has fully loaded. See here: http://jessveganlifestyle.de/beziehung-mit-fleischessern/

I can exclude this comes from an plugin, because the size is correctly adjusted when I change the theme.

Do you have any ideas what causes this problem?

Thank you!
Jess