Why is iMovie app on iPhone cutting off my videos with no crop option?

I took two videos with my iPhone (in portrait). I selected both in iMovie (iPhone app) to splice them together into one video. In doing so, the movies were auto-cropped and heads were cut off. There is no crop button to "zoom out" and pinching the screen doesn't zoom in or out. Is there any way to avoid the crop when in iMovie or is the only option to adjust video sizes in the photos app first? Also, when the movie is exported, it's just in a tiny square with a huge black frame surrounding it.