The thing is, if she's in halls then that's not really her home. I mean she stays there, but there are rules as to how she can decorate it, what she can do in it, surrounded by people she's never met before. It's still someone else's property, and she'll only be there for a few months at a time! It will feel temporary to her, that's how the cookie crumbles. At the same time, she's been pushed into the her little sister's room at her parents' house, her stuff has either been chucked out or put in the loft. It's been made clear to her that she is a lesser priority and less welcome there too. Maybe she wouldn't care, maybe she's excited about university and won't look back. If she's feeling insecure and worried about it, she may find it very unsettling to find she doesn't know where she belongs anymore.
There is not a magic switch when you turn eighteen, you still rely on your family as much as you did the day before when you were seventeen. You are still as unable/able to pay rent as you were the day before. Because she has made the choice to leave home and take a step toward her adult life, she has been told she is less relevant, and is told that it was her decision to do this, she has decided to leave so now she must deal with the consequences.
For what it's worth, I do think DD2 should have the bigger room but not until DD1 doesn't need it anymore. As long as it is a source of comfort and stability she could keep it. I'd say by New Year her old life at home will probably seem childish compared to the one she's leading at uni. When she comes home in that holiday, swap the rooms then. DD2 has waited years for her "turn" anyway, three months isn't so long now.