Site Editor: Pattern previews too small?

Question

I created a Block Theme, and when viewing my patterns in the "Site Editor", they're displayed too narrow in the preview:

When activating the Twenty Twenty Four block theme, they are displayed correctly (meaning filling the entire space).

Anyone has a clue what is missing in my block theme?

Appreciate any hint!

Answer 1

Fixed! Should anyone run into a similar issue:

In my case, I ignorantly had global iframe styles that interfered with the preview.