Crushing damage is damage done when a force interacts with the body in such a way that the body takes severe internal injuries such as broken bones or injured organs, and requires Surgery to heal. However, most Poor or Fair Crushing injuries don't present a major problem and heal on their own like Fortitude damage. It isn't until the damage becomes Good or better that internal injuries start occurring.
Small-scale Crushing damage might involve having a stone pillar fall on your leg or having someone attempt to choke you. Large-scale Crushing damage might involve being run over or being smashed between two moving walls.
If Large-scale Crushing damage is done, the GM must decide the type of injury based on the severity of the damage and where the character was crushed and treat it as multiple Crushing wounds of the same or differing severity around all affected parts of the body. Remember, damage in this game is cinematic, so the GM doesn't have to calculate out all the damage mechanically using some formula. However injured the GM says the character is is exactly how injured that character is.