As you can see from the image above, there are a bunch of killer whales in here. With so many, there is going to be major problems with finding food, creating a competition.
There are a bunch of sizes, showing a big variety from the ones who weigh 6 tons, and the tiny new-born babies, but there is an abundance.
As you can see in the image above, it is very difficult to survive with a huge population and a small food source. Also, there is a huge problem with sharks picking on the small, defenseless killer whales. Competition in the Arctic Circle is really bad.
The bigger the killer whale, the more tall genes given to the offspring, creating a bigger, stronger, more adapted killer whale. The 2 killer whales on the left made a big kid, but the smaller killer whales made a much smaller, less adapted kid.
The bigger the killer whale, the easier it is to be able to kill bigger fish, or even smaller killer whales. This means that the smaller killer whales now have a predator they have to look out for, and they have to find a limited food supply. This causes the bigger, stronger killer whales to be able to dominate the environment, causing the smaller ones to die off.
Now, there are bigger, more adapted killer whales that are on top of the food chain, and can dominate smaller, less adapted killer whales, and other species. Now, there are less killer whales, and life is more adapt. Now, they will continue to breed, creating bigger and bigger offspring.