Hi Daniel,
It looks like there’s something changing the width of default WP widgets page.
If I put your and my WP admin next to each other you can see that your widgets are double the width than they should be, see this screenshot:
https://screencast.com/t/vd1UQHL8cJFu
Here you can see that even with CustomSidebars my widgets are not changing width:
https://screencast.com/t/HW6rLs7HCup
Can you try disabling CustomSidebars on your site and see if the widgets are still wider than they should be?
If they are then try disabling your plugins one by one to determine which one is causing it.
Cheers,
Predrag