Apologies if this is a duplicate, I couldn't find it at all.
I agree with the SO/SE layouts being fixed width, the code formatting, predictability, etc. make perfect sense. However, I'm not sure this is the case with Data Explorer, I think the results pane should be allowed to expand to the width of the browser if needed.
When you're dealing with an unknown number of columns of tabular data, the fixed width seems like an unnecessary constraint that makes it a lot less usable in some cases. It wouldn't be a huge layout change visually, but it would add a lot to the usability. Expanding with the browser width as the maximum (horizontally scrolling the entire page would be undesirable), and the current width as the minimum I think would be a good set of limits for both look and maximizing usable real-estate.
Somewhat related: the date columns could have their extra width trimmed down as well to help on the horizontal scroll, this query is a good example.