The Audi Q3 has hit the sweet spot of one of the world's hottest auto segments -- compact luxury SUVs. Photo credit: Audi.

Volkswagen (OTC:VWAGY) was the most profitable big automaker in the world last year, trouncing even the two rivals that outsold it, Toyota (NYSE:TM) and General Motors (NYSE:GM). A big part of that profit was driven by VW's wildly successful luxury brand, Audi.

Audi's clean designs and top-tier interiors have made it a favorite in markets around the world -- including China, where it has become the leading luxury-car brand. Its sales have continued to grow this year, with the Q3 SUV driving big gains recently. In this video, Fool contributor John Rosevear looks at the drivers behind Audi's latest sales gains -- and at how they compare with VW's other big luxury brand, Porsche.