Not sure of the size of your image to your Video Mode resolution.
Most likely your mage is resized to accommodate either the width or height of your video mode, which Shotcut does automatically. You’ll need to use the Size, Position & Rotate filter to set it to “Fit” on your images. You’ll need to do this for every single image your bring in that is not the same resolution as your video mode it. For text images I would avoid using any zoom at all, especially increasing past 100%.
