Is time to let go, the freaking show makers confirmed that Quartz are just high ranking soldiers, Rose Quartz was a medic type, that is why her abilities tend to go towards support. She wasn't a Diamond, or part of a ruling class, she went rogue, attracted a sizable rebellion, because those walls are explained from the rebellion's perspective, because Rose was their leader is natural that from them Rose was the closest thing they had to a Diamond of their own, thus they painted with a large size. link:

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