"4th Biggest Lake" is probably to highlight the scale of the problem and make the point that humans
can screw up large areas by overtaxing the natural systems. It's doubtful most people have any idea of the size of the Aral Sea B4 the Soviets ruined it. "Seas" can be small (Sea of Galilee) or large (Caspian Sea, which is also dropping, but has a history of large fluctuations.)
Maybe the title should been: How Soviet Russia Destroyed the World's 4th Biggest Lake, the Aral Sea.