Because it was always that size. Your wound may seem bigger and deeper after debridement.
“Seem” is the key word. What has changed is what can be seen, not what is there. When a pressure sore is covered in dead skin, the dead skin makes it hard to tell how deep the sore is, and the sore is called unstageable. Removing the dead skin doesn’t enlarge the wound; it shows its true size for the first time.
It’s also worth knowing what should be underneath. After a debridement the area will be red or pink in color and look like fresh meat. A foot ulcer should be red or pink afterward, too, and wounds that are pale, purple, or black are less likely to heal.
So after debridement, color is the signal, not size. A wound that is larger and red or pink has had something removed. A wound that is pale, purple or black is less likely to heal, whatever its size.
Measuring is also how progress is tracked afterward. A wound care team will examine and measure your wound, and the measurement that matters is the one taken after the dead tissue is gone, because that is the first honest number anyone has.
None of this means any particular wound is doing well. A wound that keeps getting larger at visit after visit, after debridement and not because of it, is a different thing, and it’s one to raise.