My best guess is that they show up small because they ARE small.
You can zoom in on them, but the quality will suffer. The more you zoom above 100%, the worse it will look.
So, you have a choice, edit all of your video at the smallest frame size of all of your source material, or suffer with quality problems.
Most photos nowadays are bigger than the 1920X1080 of full HD. If you images are much larger, then you can pan around the image by changing the position. There is position and scale. You zoom with scale and pan around with position.