iMovie '11 - clips are zoomed/cropped when added to project
I have been using iMovie ('11) to process some old VHS home movies. All of a sudden new clips added to a new project are zoomed/cropped - not showing the whole of the video frame. A substantial portion is missing (outside the viewer frame). At first I wondered if this might be a stabilization effect but I am sure it is not as (a) other clips have worked fine, that are just as unstable and have had stabilization applied and (b) the zoomed frames are just as unstable (wobbly camera work by cameraman!) and (c) I imported some clips taken when camera on a tripod and it STILL crops them (though the amount of crop might be less - hard to say).
I've spent 30 minutes importing/testing different clips, have restarted iMovie and have rebooted. Still it happens.
What is going on? How can I stop iMovie deciding randomly (and permanently now) zooming clips added to a project and force it to add clips as original size?
MacBook Pro 15″, OS X 10.11