When you size down a video clip and add a “push” transitions it won’t push the clip off screen fully.
The transition occurs for the most part “with in” the sized down clip.
So how do you using a transition on a sized down clip fully move the clip off screen?
If you use TRACK motion, it shrinks the active area of the frame for that track. So if you do a transition, it only happens in the active video area (which is within the shrunken image).
Try using CROP/PAN to resize your image. Now the PUSH will push it all of the way off, but you have a black area between images.