There are under browser "settings" both font size and page zoom although those will affect all sites.
I did some testing on the wiki to see what happens at what resolution.
By sizing down browser window width I see that the wiki skin has a minimum size (~1545 pixels @ 100% browser zoom on my computer) under which, the right sidebar jumps to the left under the existing sidebar.
Reducing the size under 1270 pixels make the skin jump to a mobile format.
The forum does the same thing.
It jumps to a mobile format when the browser window is sized down below ~810 pixels.
So both sites do the same thing but at significantly different resolutions.
So we have to consider that famous phrase "It's not a bug, it's a feature" in reply to the behavior of the skin.
With so many screen resolutions out there and so many mobile devices, developers have been challenged to make web sites appear attractive and readable on all devices and in all display modes.
Your browser full screen resolution of 1024 x 768 is the trigger for this skin to go mobile. Only by reducing the page zoom will it show it's full format. Alternatively you can change the skin if you want:

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If I switch the site default to monobook, the site is very difficult to navigate and read on mobile so to gain more audience, mobile friendly skins are the only way to go. Does the "Timeless" skin need some further attention? Yes and I will continue to tweak it and read the styling sheet code to see if I can get it to stay in full format at lower resolutions.
Have you noticed over the last 5 years the amount of conversation on the site has dropped to almost zero? Thank you Facebook. It's the Walmart of the web inasmuch as it has killed a lot of smaller web site forums. It's poorly organized though for retaining subject driven conversations and photo galleries. It essentially is babble streaming. Once it's been babbled it quickly disappears replace by more recent babble.
So the point is, I want to see the audience realize that and come back here to retain the conversation that have been lost in the babble stream on Facebook. And it needs to be mobile friendly in this day or it doesn't stand a chance of happening.
By the way, last night I reduced the size of the font on the wiki skin. Did you notice that?