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.
Identical to the appearance on my LG G3 using Chrome.
My initial recommendation (for whatever it's worth) would be to put the avatars into the post headers, rather than have a dedicated column for them. Make them smaller on mobile; two lines high, perhaps?
However, I don't know if the folks making the forum software intend to do more with the avatar column.
Also get rid of the extra padding.
In a way, the adaptations the browser designers/W3 choose to try to help mobile browsing just create more problems. (Stupid viewports. :(