Look at the mirror and notice what is not there: the glass stops well short of the frame's outer edge on every side. The gilded scrollwork does not hug the mirror, it flies out past it, so the silhouette on the wall is far larger than the reflective area inside it.
That is a deliberate old trick and it solves a real problem. A mirror sized to fill its frame gives you a large bright rectangle that dominates a room and flattens everything around it. Pushing the ornament outward instead lets the frame occupy the wall while the glass stays a comfortable, human size - large enough to dress by, small enough that the room is not competing with its own reflection.
Building it that way is the hard part. Openwork carried far beyond the glass has no backing panel to hold it, so each scroll has to be planned as a structural chain running from the mirror body outward and back. Our carvers lay the whole silhouette out first, decide where the load paths run, then cut - working the outermost curls before the frame becomes too fragile to handle, and leaving the pierced crest for last.