I have the problem that the preview of my block loads very large (it seems like it grabs media query). Actually my block should shrink 1:1 in the preview, right?
Hard to say without seeing your CSS.
You can use is_admin() to output something only for the WP dashboard. You could toss in some CSS that only is on the WP admin to fix your render.
https://developer.wordpress.org/reference/functions/is_admin/