Are you talking about scaling each clip AFTER you've performed basic motion adjustments (scaling/rotation/position). If so, I don't think there's a way for Premiere to automatically detect transparent areas you've create yourself and scale each clip indvidually to fill.
Ultimately, if you're manually changing motion paramaters, you run the risk of making your clip fail to fill the frame. (ie. if I rotate an image and forget to scale it up, Premiere won't stop me for leaving parts of the frame uncovered) You could howver enable the transparency grid, or use a vivid color matte on V1 to be a clear indicator to you if you push the image too far and leave sections of the frame uncovered.
Unless I'm misunderstanding what you meant by keyframing rotation and position - in which case, if your goal is just to globally scale everything up, you can just nest everything after your edits, and then scale up your nest so that frame edges are no longer visible in your Program panel.