A catgirl that is a tank is best described as a catgirl that isn't a slinky strip of taffeta and giggles. A catgirl tank is something born in the mind of the deranged and birthed by desires foul and unreasonable. You take the ears, the tail, the nya, and you attach those dinky accessories to a woman that can overhand throw an F-150 like a paper plane and you're on the way! A catgirl tank isn't a tank in which a catgirl does battle; a catgirl tank is the tank given claws, one fang that sticks over her lip when she smiles, and a maniacal desire to be pet by anyone and everything too slow or too weak to escape when she sits. Because she'll sits where she fits. She'll sit where she doesn't fits. And when she sits, you'll know about it - that is a whole-ass tank sitting on your lap, headbutting your jaw with kiloton force and demanding scritches.
That is the essence of a Mackgirl.