In my observation, there's one line in the thread GremlinSA linked to that's causing the code block to be so wide, but, I also noticed that the code block is even wider on a larger screen, and then it goes all the way to the right when you remove the ad column - so it simply fills the available width, unless the content exceeds it, in which case it takes the smallest width possible without displaying the horizontal scrollbar. The second part is the problem.
P.S. I really think making the in-post links more distinctive is important.