picture in picture zoom to full screen

You can zoom a picture in picture by animating it with the Keyframe feature of PIP. Here's a help link that will show you how:

https://help.apple.com/imovie/mac/10.1/#/mova1aaa682b

In you case, insert your PIP with the overlay feature of iMovie. Then use the right arrow on your keyboard to advance the video by a few frames. Then resize the PIP to make it bigger and move the resized pic to a new position. Then click on the Add Keyframe button. Repeat the process by again advancing a few frames, resizing the already resized PIP to make it bigger, moving it to a new position, and clicking on the Add Keyframe button. Keep repeating until the PIP is zoomed to fill the screen. Then click on the blue reset button at the top right of your screen to set your changes.

-- Rich

Yes, it can be a bit tedious. I usually hold down the right arrow button for about a count of 7 when advancing the video. The longer you hold it down to advance the video, the faster the zoom will be as you resize the insert at each stopping point. In other words, the stopping points (Keyframe points) are farther apart and makes the process less tedious.

-- Rich

You can also zoom in using the Crop/Ken Burns Effect. In the preview screen set your start adjustment rectangle small and your end adjustment rectangle large. It's less tedious, but not as versatile as the PIP feature with respect to positioning.

-- Rich