Honestly, as an adult male (with adult male sized hands), a difference of 1 oz and 1mm seems insignificant to me. I suspect those measures are well within the variations caused by different cases. I suppose it could make a difference when the thing is in your pocket all day, but I'm not even sure about that.
The iP5 is clearly thinner and lighter, but does it matter? I don't know, not really to me. Maybe to women and kids.
Sure when you look at the mm and g it seems ridiculous but 20% lighter, 33% slimmer and stronger hardware and battery? I don't think this is SO insignificant in terms of miniaturization. They are already extremely thin and light, then managed to reduce if yet again by a fifth and a third. And the variations for their parts are below tenth of 1mm.
I definitely fumble my Galaxy Nexus on a regular basis. I can't quite accept I am unable to reach the top of the phone from my normal bottom grip so I stretch and stretch and try to balance it on my palm.
It's only my observations of the few people I have seen interact with the S3. I haven't seen anyone operate it successfully or elegantly with one hand. Often I see people set it down on a surface and use it like a small tablet. Just observation. I have only seen a few people actually using the device.
The iPhone 5 is 3.9oz and is 7.6mm thick.
The iPhone 5 is around 18% lighter and 12% thinner than the S3.
I often see people fumble with the S3, especially one-handed, so I'm not sure the size is a good thing.