Unlike the modern iMovie (version 10.x and later), which requires a hefty download, a modern Metal-capable GPU, and macOS 11 or newer, iMovie ’11 or iMovie 9.x (build 1033) was lightweight. It was the last generation of iMovie that felt truly "Prosumer" without forcing you into a subscription or a complete hardware upgrade.
macOS should recognize that your current OS cannot run the latest version. It will ask if you want to download the "latest compatible version" . Confirm this to get 10.3.3 (or whichever is the highest version your OS supports). Compatibility Overview imovie 1033 dmg best
For the highest resolution and least compression, use these settings in the export window: Select Share : Click the button in the toolbar and select Resolution Unlike the modern iMovie (version 10
Here are a few tips and tricks to help you get the most out of iMovie 10.3.3 DMG: It will ask if you want to download