But I'm wondering if I understand the space issue correctly. I thought it was mainly an issue of being able to keep warm enough, but maybe there are other reasons
It is also about the fact that the bees have to be able to patrol and defend the space when they are not clustered. A package starts out in a state of severe disadvantage. They have no food, no comb, and many times not even a queen they know or who is even related to them. This makes them very vulnerable, and if they have too much space too soon, it can simply be overwhelming for them to try manage everything they need to do to make the space a functioning nest.