I'm using a native 1680x1050 resolution on my computer and with this, Chess.com appears like a light strip with two huge black empty spaces on the sides. This doesn't look very good and can sometimes be downright annoying. Since the site seems to have a smartphone-friendly layout available, how come there isn't a layout designed for widescreen?

I agree with this.  It actually seems like a recent change, especially in live chess. 

This is what I see in live chess now:

The mobile version of the site on my iPhone 3gs with iOS 5.1 is always cut off on the right. A good page to view to test this is the tournament page where I can't view the tie-break columns or anything to the right of it.

FYI: I posted this here because I think "Site Width" is appropriate. :)

I found a solution to my problem... I was in "simple mode." In the bottom right of my pic, there's a star next to "full board." This filled my entire screen again and fixed my problem.

That may only be available for premium players and is only for live chess... not for the rest of the site.