Try validating your page with the W3C Markup Validation Service and fix the errors reported so that we can be sure that any HTML errors on your page are not contributing to your problem.
Here is the same gallery again, this time embedded in html as recommended by JuiceBox.
http://www.qr-presse.eu/e/ritz-html-embed.html
As you can see the gallery doesn't resize at all.
A Juicebox gallery will resize dynamically with the size of the user's browser window only if the dimensions of the gallery itself and those of all parent containers are expressed as percentages. If there is an absolute pixel value anywhere up the chain, the gallery's size will become fixed (e.g. 100% x 100% x 800px = 800px).
With this in mind, you could perhaps try using the baseUrl method of embedding (instead of an iframe) and use a layout which would allow this to happen.