The evidence on the screenshot only suggests to me that buckfin writes fairly short paragraphs and sometimes forgets to hit the enter key twice when starting a new paragraph.
The second thing I see is that you view the forum in an exceptionally wide window.
On a device with such a high resolution it is entirely normal to have to size a widow appropriately for the content. Such screens are designed for those who create or view very high resolution graphics. Even HD tv is only 1920 pixels wide so on your screen you could almost whatch two channels side by side without any loss of resolution.
Most authorities recommend that text should not extend beyond 8-12 words a line to keep it easy to read. That's why newspapers and many magazines print their stories in columns. Text does not need a high resolution to be readable, perhaps 200dpi so even a window 800px wide is more than adequate for viewing text and at 1024px is beginning to make text difficult to read. Many people view text on screen in far too wide a window and it is why many prefer to print large bodies of text.
The Nabble setting found at:
Options > Application > Change Appearance > Preferences > Page Layout
Is designed to control the the layout of certain apps when it is known that the majority of visitors will be using mobile phones and other small screen devices. The effect is most obvious on those such as Blog, Newspaper and Gallery that on the default Wide setting display with a sidebar but have none on the Narrow setting. There is no detection of the device used by the forum visitor. It is simply an alternative layout that some administrators choose to use.
In short, it is to be expected that visitors to text intensive sites will view them in approprite sized windows.
I see that you run Firefox. That browser makes it especially easy to view text intensive sites. You may find it worthwhile using the zoom feature. The simple way is to use CTRL-+ and CTRL-- to zoom in and out and CTRL-0 to reset zoom. Firefox will remember the zoom level usel for any site you visit. You could also experiment with the "Text only" option uner the View > Zoom menu.
One of the things that underpins the web and electronic documents in general is that they are designed to be readable by anyone in the way that suits them. Many people with sight impairments won't even view a site, they'll listen to it using a screen reader, as will people checking their emails while driving or walking round the house while giving voice instructions to their AI driven speaker.
Just a Volunteer Nabble Helper - because the nice folk at Nabble have helped me! GregHelp - Building a set of answers to Nabble FAQs.