A high capacity portable sheet cassette tray assembly for supporting and positioning a stack of sheets for feeding reliably, one at a time, in a sheet using machine. The high capacity cassette tray assembly includes a cassette frame for removing and reinstalling into a sheet supply station of the sheet using machine. The cassette frame has a front end over which sheets are fed into the machine, and forward feed corner snubbers that are mounted to the front end of the cassette frame. The high capacity cassette tray assembly also includes a liftable base plate mounted within the cassette frame for supporting a high capacity stack of sheets having a topmost sheet. Further, the high capacity cassette tray includes a constant angle parallelogram lifting assembly that has a rear end connected to the cassette frame, and a front end connected to the liftable base plate for lifting and supporting the base plate and a topmost sheet of a stack of sheets on the base plate. The base plate and a topmost sheet of a stack of sheets, as lifted and supported regardless of a number of sheets in the stack of sheets, have a continuously constant sheet feeding angle relative to a horizontal plane through the forward feed corner snubbers of the cassette frame.