Change camera zoom based on device

I’m using the Orthographic Camera library for the camera, but I’m noticing a discrepancy between HTML5 and iOS.

This is what the game looks like at the same zoom level of 2 (assuming FIXED_ZOOM is also on) on different platforms.

HTML5 desktop:

HTML5 on iPhone:

Native iOS app:

As you can see, it’s a reasonable size on desktop, but on the phone it’s much smaller. I’ve discovered that a zoom level of 4 is better on my iPhone, but I can’t say for certain which zoom level is going to work best on every mobile device.

What’s a good way to tackle this problem?