Thanks for the welcome and the help.
The Osprey looks good on paper and as luck would have it, I've just discovered whilst searching this morning that a new place has opened up 2 weeks ago near us that stock this so we can see it in the flesh. I'm going to pick her up after school today to check it out.
You're right about cost as I've found out but I would very much like to encourage her willingness to embrace outdoor pursuits so it's something I can justify in my own mind. (Also selling things on eBay helps, lol!) Things are always much more positive if they don't involve the kind of pain, struggling and hardship that can come from ill suited kit.
You also may be right about underestimating sizes/weight etc but I've become somewhat obsessed with the subject and I'm doing lots of research on new kit. I've just got a (need to whisper this because of the cost) Sea to Summit Spark SPIII sleeping bag that weighs 640g and compresses down to the size of a grapefruit. It's treated down and my first impressions of it are good. Ultra lightweight, very soft and comfortable and very warm. I've also changed her cutlery and got her a titanium spork which saved another 40g. I've even swapped out her emergency ration pack and got lightweight jelly beans as one item instead. See...obsessed. There is not one area of her kit that I haven't looked at to see if there is room for improvement.
Anyway, we'll look at the Exos to see if we can judge which size would be best to go for but if the sizes all compress down anyway and they are all light, then perhaps we'd be just as well getting the larger version. I'll let you know how it goes.
Thank you.
And thanks to everyone else who posted to help too.