If the ecosystem is balanced, which populations should be the largest? which should be the smallest?

There would be more prey than predators, as for a predator to have a balanced diet it should have a plentiful supply of food, meaning it needs more prey. there would be less predators because there shouldn't be so many they would outnumber the prey or so little they can't repopulate and die out.

Nitrate poisoning for the first onebaby blue syndrome

The answer to your question is vagina