Succinctly, that's exactly the confusion I was trying to express. "Compounding" a steam engine (i.e., using steam over and over again until its pressure is thoroughly exhausted) is a common device in large steam engines, and every engineer learns those techniques. They're more appropriate to large ship engines -- e.g., the "triple expansion" designs, but I was aware the principle had been used in steam locomotives. I just wasn't exactly sure to what extent.
That the Big Boy was articulated goes without saying. You can't wrap something that long around a curved track without it. I knew for a fact of the articulation, but I was unsure whether the Mallet design required both the articulation and the compounding. Hence my guess as "Mallet" (for the articulation) but not the traditional kind (because I wasn't sure about the compounding).
