I have a responsive email that starts at 645px, and adapts down to a 300px wide version for mobile.

I'm finding that my 300px mobile version is not filling the full width of the recent iPhones. It fills the screen of an iPhone5. On an iPhone 6 it has side gutters of 15 px. On an iPhone 6+ it has a 30px gap on either side.

I understand that iPhones auto zoom into your email so this may be a limit of the client.
I also understand the recent iPhones have bigger screens more resolution, and therefore may refuse to zoom in anymore since zooming into a 300px wide email is not ideal.

In testing I found I can only overcome this by increasing the overall width of my mobile email. I think I have to up the width to 400px to fill an iPhone 6. But even at 400px it still still leaves side gutters on the iPhone 6+. I think I have to increase the width to 460px to fill the iPhone 6+.

So am I missing something or is increasing the size of the smallest responsive email my only option to fill the biggest iPhone?