For example, if running at 320x480, the "Members Online" box is 300px wide. If I then resize to say 360x640, that same "Members Online" box becomes only 250px wide. This seems to occur at viewing widths 340px or larger.
The same thing also occurs if switching to landscape view. The sidebar drops down to 250px wide.
I am seeing this on several mobile devices, and also using Firefox responsive mode developer tool.
I'm not sure if this is a bug or intended, but it is causing me some issues where I have 300px ads in the sidebar.