At the bottom of this document it talks about how much krill these things would have eaten. Krill live at the surface, perhaps 1 meter, so 15 billion cubic meters of ocean is something like 15000 square kilometers, which is the area of maximally krill-swarmed antartic water that the blue whale population would have filtered through each year, before we killed nearly all of them. I don't know how fast krill populations reproduce, but it seems like that's enough consumption to materially affect the local environment.
Compare 136 million metric tons of krill per year eaten by all those whales (blue, fin, humpback and sei) to about 700 million metric tons of oil a year consumed by the United States. Obviously krill don't have quite the energy content of crude oil, but the notion that the numbers are even comparable is just boggling.
The document suggests that krill have energy content of 3.8 MJ/kg, which is considerably lower than the approximately 25 MJ/kg of crude oil. Each day a blue whale would eat 3 tons of krill and gain 400 kg, so if the weight gain was mostly fat and not water, the whales would have to be converting nearly all the swallowed krill energy into fat. Later that would be burned off into CO2, so my guess is that these whales were gigantic hydrocarbon burners, consuming energy equivalent to 3% of U.S. oil consumption.
Almost all those animals are gone now, so I wonder what is happening to all those krill down near Antartica right now. Nature abhors a vacuum.