Small elements in the frame tend to have low "aesthetic energy," while larger elements tend to have high "aesthetic energy." This means your audience will not only pay more attention to the larger elements, but their eye will see them first before anything else.

The purpose of a frame is to focus your eye. To focus the eye of the viewer, so that your peripheral vision isn't distracting, doesn't distract you from the content. Again, the purpose of the frame is to keep your eye on the content of the frame.