I think this is just the nature of the H5P plugin.
I use a MacBook Pro and Safari as main day to day laptop, and I often have to go into fullscreen mode to edit at a reasonable size.
Note: I have also seen the "Exit fullscreen" button completely disappear !
Although, if you know where it is located on the screen, you can actually click it
seems to work better.
I can't see there is much we can do about this, unless you wanted to start styling the Hr5P plugin with CSS
And that probably isn't necessary, with the H5P integration planned for 3.9 not too far away now.
I certainly hope, and in fact expect, that H5P activity interface will be a lot more "Moodle'y"