A world with a very large number of individuals whose welfare levels are barely above zero can have a larger sum-total of welfare than a world with a smaller number of very well-off individuals.
Parfit (1984) and others argued that classical utilitarianism is subject to the repugnant conclusion: a world with a very large number of individuals whose welfare levels are barely above zero could have a larger sum-total of welfare, and therefore count as better, than a world with a smaller number of very well-off individuals.