@Pangloss,
Sounds like she should pay more, given the size of her room.
Mostly, though, this sounds like one of those gray area decisions. Is it worth arguing over? I can't say, nor can anyone else here. Is it fair to split the bill 50/50? I can't say, nor can anyone else here. These are judgments about which you know infinitely more than any of us.
From what I can tell, she should pay more. I'd bring it up. Though, I'd also consider that she has contributed a great deal to the apartment. Maybe suggest 60/40?
Good luck, whatever happens. Just remember, college is a time for fun. For living. For meeting new people and enjoying their presence. Stand up for yourself, but don't risk the good things.
The ethical thing to do is to not make an issue out of money.