Use different scaling on tablets


I’m writing an app which is primarily aimed at phones, but I’m trying to at least not have it look ridiculous on tablets, so I simply want to scale things up by roughly a factor of 2. Using relative units for everything isn’t really an option, because I have several somewhat complex html constructs which would make this very difficult. So I’ve tried to use some javascript code in the head of index.html to replace the viewport tag with one at a different scale, but somehow this isn’t working on the tablet I’m using for testing (Galaxy Note 10.1, running Android 4.1). On my phone it works.
So I was wondering if there is some way to for example use a different index.html file depending on device type, or if anyone can think of a different way to achieve this. Creating separate APKs just for that seems like complete overkill to me.