I have just tried embedding a gallery on Google Drive at less than 100% x 100% (at a fixed size of 600px x 400px) in a web page setting showSplashPage="ALWAYS" and trying both expandInNewPage="FALSE" and expandInNewPage="TRUE". The gallery expands correctly in both scenarios on my iOS device and in desktop browsers.
Here it is with expandInNewPage="AUTO":
https://7e149673cba79128b4d3fe7ac7a9b89 … index.html
I get an error 400 when trying to switch galleries on your web page (as well as when trying to expand a gallery from the Splash Page) so I think the problem might have something to do with the code on your web page and the way that your galleries are embedded or loaded.
As the problem happens when clicking one of your gallery links, you should have more control in debugging the switching of your galleries than the expanding of the Splash Page (although, being that the error messages are the same for both scenarios, they might be related to each other).
I would try debugging your web page's JavaScript code to ensure that all your variables are set as you expect them to be.
This should hopefully help to track down the cause of the problem.
After setting a variable but before using it, use a JavaScript alert box to check its value to see if it is what you expect it to be or if it is perhaps undefined for some reason (maybe due to a limitation in using Google Drive rather than a regular web server).
Check that everything is OK before running your new juicebox(content.options); line of code.
I would also try stripping back your custom JavaScript code so that only basic functionality exists.
You could then reintroduce elements, one at a time, to see if you can find the cause of the problem.
I realise that this does not directly solve your problem but I hope it at least points you in the right direction.
You might also like to look into the possibility of hosting your web site on a regular web server rather than on Google Drive.
Juicebox does not require any special server requirements so most web hosts (even those offering free web space) should be suitable.