I think your attached image does not show, how your Scene Capture Component in this view actually works. If I understand your setup correctly, your SCC should be attached to your portal itself, so it will actually capture “through the eyes” of the target portal, and not through your eyes, like in your image. However, you’re right, that there is a lot of overhead due to the relatively small screen size of your portal compared to the capture size behind it.
If you somehow want to optimize your screen component capture efficiency, playing with your render targets resolution might improve results. I don’t think, that you can tell your Scene Capture Component to somehow “do not capture” specific parts of your scene, at least I’m not familiar with a way to do that. 
Especially since you would have to adjust this area each time your player moves and it’s view angle changes.