Vegas “sees” from the top down. So what it oon an upper track will be over the top of a lower track. If you have your set up so that the smaller PiP is in the upper right corner it must be on an upper track. How did you make it smaller? Just reverse it to fill the frame. Either evemnt pan/crop or track motion. When you have the upper track filling the frame then split the lower track and move the remaining video to a track above the previous top track (You will now have 3 tracks. Resize it to go smaller and into the corner, probably using track motion. You can place a transition on the overlap area for variation.