Is there a trick to getting Outlook for iOS to display a responsive email at 100% width?

I have a responsive email that renders correctly in both iOS native mail and Gmail on Android and iOS. In all of these clients, it fills the available width. In Outlook for iOS, it's using the responsive styles (e.g. fonts get larger, columns stack, etc.) but it only fills half the available width, possibly even less.

I noticed that the Litmus Weekly from 1/29 exhibits the similar behavior. Here's a screenshot.

Anyone know how to fix this? Thanks!