Full frame can mean that your camera sees and records the full amount that the lens sends it, which is nice. There is no internal cropping of the image.
Full frame can also mean you’ve used your whole frame. You’ve exploited every square millimeter and filled it with interest.
It is not unusual only use part of frame. We leave dead space that really isn’t of any value. The most common deadspace is that space we leave above people’s heads when we center their faces in the middle of the frame. That’s easy to do. That is where we often focus.
I like to think of my frame as real estate. I don’t want to waste any of it.
I guess a better term would be “Fill Your Frame”