Yea, I was going to try and scale it accurately, but the S3 is in the foreground, so there's perspective to deal with as well -- so I decided to just leave it as-is. Maybe I should've put 'image is for illustration purpose only' at the bottom...
You can't possibly put the Galaxy S3 in front of the iPhone and create that scene. That picture is a mess.
iPhone 4/4S: 115.2 x 58.6 x 9.3mm
Galaxy S3: 136.6 x 70.6 x 8.6mm