The quickest & easiest way to scale – “zoom” – a video clip in iMovie HD is to export the clip, do the scaling in Final Cut Express, and then re-import it into iMovie:
In iMovie HD
Select the clip in the Timeline that you wish to scale (you may need to prepare a duplicate clip first, crop it to the right length, and place it in the timeline).
Go to File > Export… In the dialogue box choose the QuickTime tab; select Full Quality from the drop-down menu; check the box marked Share selected clips only; click Share. Give your clip-for-export a suitable name, and save it somewhere you’ll remember. Minimize iMovie.
In Final Cut Express
Go to File > New Project; a new ‘blank’ project will open. Go to File > Import > Files… and choose the clip you exported from iMovie.
In the Browser window, double-click on your imported clip: it will appear in the Viewer window.
Go to View > Image+Wireframe: a large wireframe cross will appear, overlaying your clip in the Viewer window.
Roll over the picture in the Viewer with your mouse: the cursor will become a cross with small arrows on each end.
Roll to the top right corner where there is a small white dot; the mouse cursor will lose its arrows and become a simple cross-hair (NB. beware, there are several other cursor variations!)
Click and hold on the small white dot with the simple cross-hair cursor; drag to the right and the clip will increase in size. Release the mouse when you have the result you want.
Go to File > Export > QuickTime Movie… Give your modified clip a suitable name, and save it somewhere you’ll remember.
Quit from Final Cut (choose No to saving any other files or projects)
In iMovie HD
Go to File > Import… Select your modified clip and it will automatically load itself into the iMovie clips bin.