When the footage was imported, aspect ratio was probably set to 4:3. This is just metadata that does not need to be rendered.
If you use resize on the editdesk, it will render. However, in the library, you can apply the change instantly by using Tools/Reformat (as Andy alluded to). As long as the only change is the aspect ratio, it will not render. A resolution/bit depth change will force a render.
Also, in the library, you can use Resize on Load. Again, if the only change is the aspect ratio, it will load the clip to the editdesk with the change without rendering.
Now the proxy thumbnail will be the correct aspect. Alos, if you enable "use ratio" in the player, the aspect ratio will look correct.
cheers
Jason