So here's what the forums look like on my phone without any custom CSS:
The avatars are extremely large compared to their pixel size on desktop (64).
Also, the items below the "post-content" div have picked up padding-right:35px; somewhere in the recent updates, resulting in a huge gap on the right.
For reference, my phone is a Nexus 5 (1080x1920 in portrait) running Android 6; the browser shown is Opera Mobile. The image links to an uncropped full-size screenshot.