right sidebar doesn't shrink in zen responsive theme
Inspect the right sidebar element in your browser. Then select 

&lt;section class="region region-sidebar-second column sidebar"&gt;


in the DOM and it should give you the width as a percentage. This is proof that the sidebar is responsive. Now drag the browser window smaller and observe the media query changing at breakpoints and the percentage should also change. At mobile breakpoint the sidebars should be 100%.

Now look in your sub theme's sass/layouts folder for the file _responsive.scss

You will see snippets like this

$zen-column-count: 3;
...

#content {
          @include zen-grid-item(2, 1);
        }

        /* Span 1 column, starting in 3rd column from left. */
        .region-sidebar-second {
          @include zen-grid-item(1, 3);
        }


This is where the sidebar width is set in relation to the content area. In the above case it is 1 column wide starting at column 3. As there are a total of 3 columns it is 1/3 the total width.

You can change it here if you have compass set up and working and properly watching the folder for changes.