I think most of us are looking forward for the improvements in the Responsive Editor.
To collapse the height we can use the UI Builder and conditional formatting using the page width option, however to collapse the width we need to use the Responsive Editor and respect the width of the container or parent group.
My suggestion is to add the page width option to Add hide rules and Collapse margins
Under Add hiding rules there could be a dropdown list with two options, Page width or Parent width, and in collapsing margins another dropdown list with, Page width or Container width.
Hopefully we can have something like that when Bubble releases the responsive enhancements.