"I do care about everything that exists. I am not particularly certain that all mathematically possible universes exist, or how much they exist if they do."

I was confused by this for a while, but couldn't express that in words until now.

First, I think existence is necessarily a binary sort of thing, not something that exists in degrees. If I exist 20%, I don't even know what that sentence should mean. Do I exist, but only sometimes? Do only parts of me exist at a time? Am I just very skinny? It doesn't really make sense. Just as a risk of a risk is still a type of risk, so a degree of existence is still a type of existence. There are no sorts of existence except either being real or being fake.

Secondly, even if my first part is wrong, I have no idea why having more existence would translate into having greater value. By way of analogy, if I was the size of a planet but only had a very small brain and motivational center, I don't think that would mean that I should receive more from utilitarians. It seems like a variation of the Bigger is Better or Might makes Right moral fallacy, rather than a well reasoned idea.

I can imagine a sort of world where every experience is more intense, somehow, and I think people in that sort of world might matter more. But I think intensity is really a measure of relative interactions, and if their world was identical to ours except for its amount of existence, we'd be just as motivated to do different things as they would. I don't think such a world would exist, or that we could tell whether or not we were in it from-the-inside, so it seems like a meaningless concept.

So the reasoning behind that sentence didn't really make sense to me. The amount of existence that you have, assuming that's even a thing, shouldn't determine your moral value.