Stockiness is the noun for the quality of being stocky — the state of having a short, thick, solid, strong-looking body. If 'stocky' describes a person and 'stockily' describes how they are built, then 'stockiness' is the thing itself, the quality you are pointing at. You use it when you want to talk about that body type as a quality rather than attach it to one person: 'the stockiness of the breed', 'his stockiness made him hard to knock over'. That last example shows where the word is genuinely useful — when the shape has a consequence. It comes up in writing about dogs, horses and cattle almost as often as about people, because breeders talk about body shape all the time. It is not a common word in speech; in conversation people just say 'He's stocky.' Like 'stocky' itself, it is neutral to slightly positive — it says solid and strong, not overweight.