I actually figured that this is a CSS bug, and here is how I patched it for now:
attInspector.deleteBtn.domNode.classList.remove("atiButton");
The problem is that in the the CSS class atiButton has a fixed 16px size (geared towards the navigation buttons) and the delete button should not inherit from it, or better yet, the class should change and a new atiNavButton should have the fixed size, etc.