I think the headline's a little misleading, my immediate thought was "AmigaOS" too. Changing it to "Individual icons dynamically sized" would probably be more descriptive.
Interestingly, it is an extension of an unrelated Amiga concept. The Amiga allowed files to not have icons at all. From AmigaOS 2 onwards, files with icons were displayed by default, and files without them were only shown if you selected "Window->View->All Files" from the menu. Generally, user created folders and documents had icons, whereas less important files didn't.
How is this related? Well, in practice, despite its origins as a disk saving hack, it made it easier to identify important files. Generally, the applications you'd want to run had icons. The word processor documents you wanted to open had icons. Things you needed had icons, the others didn't clutter your desktop unless you needed to see them.
And in that respect, I think that's the intention here. Your own files, representing stuff you work on every day, will have large icons. The metric butt-load of downloaded PDFs, application installers, etc, you download from the Internet will have much smaller ones. Stuff batch downloaded from your digital camera will appear, but it will be assumed you actually just want to see them in iPhoto, not your file system.
It's a much more spacial approach than the Amiga's (which isn't surprising because it's actually had some thought put into it in terms of "How is this useful", as opposed to the Amiga's "Damn it, the developers aren't giving icons to every file" rationale, that ended up being accidentally a really good thing.) But... therein lies the problem. If it's spacial, will it stand much chance of appearing in a modern Finder?
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