I agree, but is this the right comparison? Isn't this framing obscuring the fact that in the trillion-people world, you are much less likely to be born in the first place, in some sense?

Let us try this framing instead: Assume there are a very large number Z of possible different human "persons" (e.g. given by combinatorics on genes and formative experiences). There is a Rawlsian chance of 1/Z that a new created human will be "you". Behind the veil of ignorance, do you prefer the world to be one with X people living N years (where your chance of being born is X/Z) or the one with 10X people living N/10 years (where your chance of being born is 10X/Z)?

I am not sure this is the right intuition pump, but it seems to capture an aspect of the problem that yours leaves out.