Looking at jpeg.js, I'm confused about what it does for writing thumbnails.You might ask Diego about that. The worst thing would be for it to try to decode the full-size image and create a thumbnail. We can't afford the memory for that.

The best thing would be for us to use the most recent image preview (already created in the ImageEditor) as the preview image.

The easiest (and completely acceptable) thing would be to just not include any EXIF preview at all.