Where's the aether? (Spoilers)

[TBerry]

This does bother me to a degree. Why aren't people on the First smaller? Why is there not a clarification that using magic is harder for them due to lack of aether?

If you split STEPHENSON 2-18 into 13 evenly massed stars, chances are they are still bigger and more massive than our sun. Probably even enough mass to create 13 black holes.
It's probably enough aether to not make any difference at all.