The basic gist of iso levels is as follows.
For a given amount of light hitting the sensor (metered) an exposure duration is determined.
Increasing the iso level decreases the exposure time for the same amount of light.
A shorter exposure time reduces the signal to noise ratio significantly, but it also reduces motion blur. Noise can be post processed, blur not so much.
My old nokia n95 would have exposure times of over 2 seconds in low light conditions, chance that everything remained totally motionless in this time is very small!
The exposure duration is usually stored in the exif file, I hope her original files kept all this sort of info!
The xs' camera has potential because we can change the target iso value (though compared to a still camera they're still low) we can also change the metering method as well. I haven't seen either of these features before in a phone camera.
The only thing we are missing is the ability manually specifiy exposure duration.
You are also correct in that the higher pixel density of a sensor the higher probabiltiy of noise.
Ability to take raw photo's or 100% quality jpegs would be nice (people have done mods to enable 100% jpeg quality, but I cant root my phone at present to test it as i use sky go)
Edit - Just checked the exif info on the 'original full size' images and its been wiped
file sizes are interesting though.
GS3 - 2.1mb, IP4 - 2.6mb, 1X - 1.5mb, XS - 2.0mb
Have just noticed all the 1x pics are 6 megapixel images in 16:9 format which seems suspicious to me. But it says it all for the XS when it has a smaller file size than the GS3 and IP4 yet 50% more pixels.
Rough rule of thumb is that you need at least 1/2mb per megapixel to avoid any degradation on a full image (ie no patches of black or white)
