Hey @Zache — I agree that we should offer the ability for people to toggle the content width between a more narrow and more wide option. In terms of how we might achieve that I think there are at least two ways:
1) a bit further along in the project our team is going to look specifically at the use case of people with large screens. The same way that we want people with small screens to have a good experience, we also want people with large screens to have a good experience. There's a chance that at that time we would work on the type of toggle you mentioned
2) a developer could certainly build this type of functionality as a gadget. I think it would be great if this happened sooner rather than later because it would help us understand how many people would want/use this kind of thing, and could help with our own prioritization process. And even if we didn't get around to building a more "official" version of the feature it would at least be there for people who know how to use gadgets.
Also, to keep in mind, we are still developing an improved table of contents experience and there's a chance that it will sit to the left of the article (on LTR languages). So from that perspective it might make sense to wait a bit longer before developing a gadget to toggle the content width.