So the blue whale got big to eat more krill- does that mean that there's no other animal than the blue whale that can eat as much krill as a blue whale?
In that case, what would happen to the krill if the blue whale went extinct? Would its numbers grow uncontrolled until it took over the oceans? Or is other predation enough to keep its numbers down?
What is not mentioned in the article is the reason big animals exist: efficiency. The physics are simple: assume animals are ball-shaped (true to within a factor 2 for mammals). That means the amount of filling grows with the third power of the radius. The "skin", or outer boundary only grows with the second power.
"Skin" is how an animal loses power (by far the biggest power draw inside mammals is keeping the body warm. Obviously in water this is an even bigger problem). "Filling" is how an animal generates power.
That means that smaller animals cannot hope to match the whales in efficiency: for sustaining the same biomass, they need to keep a bigger "skin" warm (add all the skin of all individuals together).
Now when does this matter ? Obviously this would matter a lot in terms of scarcity, at which point it would be much easier for whales to survive than smaller fish. When there is richness of food, smaller is better, as you can move faster and "eat" the available energy faster. However if smaller animals do this to the point that food becomes scarce, they've just screwed themselves.
So you see what happens : the longer the biosphere goes without a major disaster, the bigger animals get (and plants, for that matter). The advantage of being the first mover, where you can be small and still win, disappears, and the advantage of efficiency grows and grows. So when food became scarce, whales, the biggest animals that had the easiest time surviving at that point, became even bigger. That made the difference between different whale species.
This framework allows answering your question: no, if whales went extinct fish would increase in numbers (slightly, in the sense that it'd be less than the biomass of the whales that disappeared). There isn't enough predation to replace the whales at the moment, but that would be very quickly rectified if something happened to them.
The real reason krill is increasing is the slight increase of co2 caused by humans (krill eat plankton, as do most whales). It's tiny when measured compared to total volume, but if you look at it in relative terms co2 has increased quite a bit (amount now vs amount 100 years ago). In reality the plant growth in the world is hampered significantly by the amount of available co2, and the fact that plants use it to store energy, a part of which gets buried and is lost to the depths (literally in the case of krill).
In that case, what would happen to the krill if the blue whale went extinct? Would its numbers grow uncontrolled until it took over the oceans? Or is other predation enough to keep its numbers down?