How to adjust the "scaling" of the image / div in the jQuery mask area?

I created this piece of JS Fiddle using some of the tips I found here in StackExchange. It works fine, but I would like to create a slider or buttons to adjust the "scaling" on the draggable div, from the maximum value that you see here to 100%. I can’t find a way to do this with Google, all methods are not applicable here.

You will notice this is not just an image, there will also be DIVs (yellow box in my example).

Here's the flash version of the effect I'm looking for.