ha. pretty much right on.
big mtn includes varying snow conditions at very high speeds. cliffs, sometimes huge. and generally giv'in 'er.
powder usually implies slower speeds, deeper snow, more trees, less varied snow performance.
big mtn skis: long, stiff, conventional sidecut, usually have regular camber.
pow skis: shorter, softer, sometimes have crazy sidecut (i.e. pontoon, arg, hellbent), sometimes reverse camber or rockered design.