Answer: The population size of the krill would decrease.
Explanation:
A food chain is a series of distribution of food and energy when one individual is consumed by the other individual organism. A food web is a complex interconnection of multiple food chains.
According to the given food web, the population size of the krill would decrease if the population of the penguin increased dramatically. As Penguin is the tertiary consumer which is dependent upon the population of krill in the food web for food.