You might need to read up on the repugnant inclusion. It compares a small population of high individual welfare with a large population of lower welfare. If you

You might need to read up on the repugnant inclusion. It compares a small population of high individual welfare with a large population of lower welfare. If you assume the desirability of each option to be the product of individual welfare and population size, each population of high welfare has an equally appealing alternative population of many people living just above the point of not wanting to exist. Utilitarianism requires additional assumptions to decide between many people in subsistence and few people living carefree. You are introducing (I suspect) such an addictional constraint, comparable to the Pareto criterion that forbids making individuals worse off in a trade off. However, accepting/demanding/proscribing such trade offs that accept making someone worse off are central to most utilitarian theories.