While working on the MDL-78096 improvement, when I tested with the below image

FF and Chrome have different results.

With FF, when you edit the image, the Original size is checked. Unfortunately, Chrome is not. When you edit the image, Chrome will check the Custom size.

FF	Chrome

The expected result is both browsers must have the same behaviour.

FYI. TinyMCE Image will add the class img-fluid to ensure the image does not break the layout.