Don't resize image, image sequences or videos when importing
When importing data into the timeline, it is resized to whatever is the video size. This is very bad default. It should be imported as whatever size it is (i.e. bigger than the output size if the input data is bigger, or smaller as the output size if it is smaller).
Of course, there could be a feature to "fit" the input to the output, but this should not be the default, because that's basically destructive.
When you edit animation for instance, you may import images bigger than output (for instance for panning through a landscape) or you may import smaller images (characters or whatnot).
Even on non-animation video edit, there are various use cases where you don't intend your input data to fill the whole screen. In any case, no resizing should happen without explicit request.