OS X Yosemite iPhoto 9.6 Referenced libraries

However, when you uncheck the tick box, and import photos, iPhoto still creates a Preview of the original in its library, and the preview is about 70% the size of the original (i.e. its still taking significant disk space).

iPhoto cannot work without the preview images. You cannot disable this.

If you really need and want to work with referenced images and store the originals outside the photo library, upgrade to Aperture, while it is still possible to buy it. For several reasons:

• iPhoto has no options to repair broken links to your referenced originals outside the library. If you need to replace a drive or want to move the files for other reasons, you are stuck and will need third party tools to rescue your iPhoto library. Aperture supports referenced images well. It has been designed to work with large, distributed photo libraries.
• In Aperture you can also disable the creation of previews or create them selectively only for your most important photos that you want to use in other applications.

Aperture can open the iPhoto libraries and vice versa. Aperture 3.3: Using a unified photo library with iPhoto and Aperture

It's not a copy (you can tell that as you say yourself it's about 70% the size of the original), it's a Preview and that's what iPhoto uses for sharing via Media Browsers.

The reason you're seeing this is most likely that your images have Exif display tags - like auto-rotate, for instance. iPhoto generates the preview as otherwise when shared to, say an email, or uploaded or used in a Word Processing document, it would arrive at the destination sideways, as these applications don't understand Exif tags.

You cannot turn off previews in iPhoto, but you can in Aperture, and have to export everything you want to share.

If you're worried about disk space the best solution is a managed library on an external disk.

For the sake of folks who may read this post in the future, here's why running iPhoto in referenced mode has many pitfalls.:

https://discussions.apple.com/docs/DOC-6361