Original and edited video changes default playback size

When I use my M2 Mac to play back video copied from my iPhone a double click expands it to fill the width of the screen with grey/black bars at top as the screen ratio is different to standard video. I then use that video in Final Cut and save the edit by selecting to save in original resolution. When I then playback the edited video a double click expands the box/window to fill the screen but the video only expands to what looks to be its pixel resolution so it has large areas of black all around it.

I have tried different renders other than just use the "as original" option and it is exactly the same. Playback with different apps has the same problem. All I can do it select zoom and manually expand the picture but it is a messy solution and I still cant see why it will not fill the screen by default as does all other video I have from different cameras and phones.