If the "Fit window" option is on, and the target FrameView size is smaller than the emulated device screen, the page contents can be downscaled too much, occupying half of the FrameView width (or even less). This used to work fine originally, but seemingly got regressed with recent style resolution or reflow-related changes.
Created attachment 140238 [details] Patch
Committed r116116: <http://trac.webkit.org/changeset/116116>