Galaxy SIII only showing half the resolution

I have a HTML5 game which is designed for 640 or wider (iphone). On any screen wider than 640, it looks good too.

On SIII mini, which is only 480 wide, its a bit cramped, but playable.

But on the SIII (android 4.1.2 + chrome) which is 720 wide, I only see a tiny part of the game, looks like its a phone with only 360 with (like switching retina mode off on the iphone).

However, an older SIII, with android 4.0, and the android browser, looks great! The game can use the full 720 width.

So the question is, does anyone know whats in 4.1.2 which is "downgrading" the resolution to half the native resolution? Anyone seen this?

The C2 project is a mobile one with crop fulscreen mode and "use ios retina display" set to "iphone only"

It really sounds like it's rather Chrome that's downgrading it for some reason. Maybe some kind of mismatch or bug.

Maybe try with another mobile browser on the SIII to see if the problem is browser-based or not?

Ok, the problem seems to be that SIII 4.1.2 browser reports 360 as the width and device-width, but has a pixel density of 2. This is more or less the same as iphone - it reports half the resolution, and you have to factor in the pixel density.

The question is, how do I hack the output of C2 to work on this device? I dont know much about CSS, but I guess the crux is to

a) detect this case

b) force C2 to use the native resoution.

Anyone done this?