Why do fish and frog lay more number of eggs whereas cow and human beings usually give birth to only one at a time?

External development of young ones occurs in case of frog and fish due to which its protection from the surrounding decreases. So to cope up with the loss they lay more no. of eggs because not many have a chance to survive. Whereas in case of cow and human beings development occurs inside the mother’s body and the egg is protected. So they give birth to only one at a time.