Is the cassowary the second largest bird?

It depends on what is meant by "second largest", and whether this refers to height or weight.

The emu is the second-tallest bird, averaging 1.6 to 2 metres in height. Its nearest rival is the Southern Cassowary which averages 1.5 to 2 metres. Next to the emu, it looks distinctly shorter and stockier. The Southern Cassowary, however, tends to be heavier. Emus range between 30 and 45kg in weight, whilst the Southern Cassowary averages 50-60 kg.