Oh! I see the confusion - sorry. The short answer is it is driven by the parent div that contains it.
If you adopt this sort of approach to anything using floats, just be aware that percentage measurements are a total of all attributes of the box model. Margins also influence total width and height. Padding does not influence percentage width or height as it is inside the box in the version of the box model Webflow employs.
So margins have to be factored into your totals or they will break the wrap.
Here is the much longer answer:
I build my own simple columns using nested divs. All of my responsive images are set to 100% width and height so they are drag and droppable to fill anything you poke them in. Their size is structured / managed by the div that contains them. Look at the navigator to see the structure and the settings to see the width attributes applied and you will see the percentage widths on the parent divs driving the actual behavior of the floats.
I also need to rebuild and republish this file ,as the class names are crazy confusing. I published this before Webflow restructured the class selector for globals, so names are apparently different in different views of the same object? Sorry for the added confusion that causes.