Don't worry, though. The phone still feels premium, even more so than the S3 or I'm guessing the S3 "mini" that is supposedly coming out next week.
I have the iPhone 5 (white). It is a beauty. It sparkles in the right light, the aluminum back has a nice feel, and the proportions are nearly perfect. Steve Jobs would have been proud of what Jony Ivy and co. came up with.
aluminum is not nearly as dense as glass or stainless steel, and since it's unibody, you need less of it to maintain the structure. i would hazard that the front and antenna glass is a tad thinner as well.