The chip - 4 stacks of HBM2 takes up what 367.97mm2? of the area of the interposer alone. Which wouldn't leave enough space for the rest of the spec if the whole package was 600mm2.
600mm2 is the GPU chip. Just as 597mm2 was Fiji's GPU (not interposer size with HBM and GPU).
IMO they likely have some serious density issues. TSMC 16nmFF is supposed to be quite a bit less dense than Samsung 14nmFF, and they may have had problems re-adding all the stripped hardware features of Kepler / Maxwell. It really shouldn't be that big.
Also, new process chip of that size ... it's going to cost them an absolute fortune, and will probably have gigantic defect rates.