So more generally, I’d say that population ethics is the study of how to compare the moral value of different populations.
As an aside, even if I agreed with that definition, I don’t think infinite ethics would be a subset of population ethics.
The distinctive problems of infinite ethics arise roughly when we can affect an infinite number of value-bearing locations. But this is independent of what those value-bearing locations are—in particular, they need not be people.
For example, we’d run into infinitarian paralysis if we thought that each of our actions affected the axiological value of an infinite number of paintings.
As an aside, even if I agreed with that definition, I don’t think infinite ethics would be a subset of population ethics.
The distinctive problems of infinite ethics arise roughly when we can affect an infinite number of value-bearing locations. But this is independent of what those value-bearing locations are—in particular, they need not be people.
For example, we’d run into infinitarian paralysis if we thought that each of our actions affected the axiological value of an infinite number of paintings.