RPS will have the data on their visitors, so only they will know specifically.
But i assume that Discourse will also have a fair bit of data about what resolutions people are using, so if they’re targeting specific widths with their layouts it seems likely that they feel that’s a common width. (and most other sites seem to agree).
To be clear, I’m not saying that it wouldn’t be nice to have a wider layout for those that would like it, just that I felt the forum was getting a little unfairly criticised for something that seems pretty standard among many sites.
I don’t use the forum much on mobile, but (as long as you’re signed in) I’ve gotta say it’s a huge improvement over the old forums on that front. (Not that that was hard)
 Random aside that Graham won’t be happy to hear:
Since the new forum came online I’ve basically stopped visiting the main site.
I’d been visiting less over the past year anyway, but I still tended to check it pretty often. I think I’ve only checked it twice since the new forum came online.
The reason is very simple, and shallow. There is no link to it.
I always used to check the forums first, and then click on the link at the top to check the main site. Now I have to retype the address, and it turns out I’m just too lazy to do that.
And I just realized that the advent calendar is in full swing, so I need to go and read those now.