By default, Aperture automatically generates half-size JPEG previews of your photos for use in other applications. Whenever you drag an image onto an email message, desktop or office document, you’re not getting the original size & quality. Instead you are getting the preview image. In most cases that’s fine, because you don’t wont necessarily need the image to be full size. However, it’s worth knowing that you can change the size if you wish to.
To change the photo preview size:
Choose Aperture > Preferences from the menu bar, and click Previews.
From the Photo Preview pop-up menu, choose a size option, or choose “Don’t limit” to generate full-size JPEG previews.
Note: To update the preview size for existing photos, you must secondary-click, hold the Option key and choose Generate Preview.