It's quite hilarious that so many people *know* so much about HDDs, but none can ever seem to explain the differences in throughput with any real granularity other than "higher-density platters are faster"
I think of that as a simple approach to understanding. Do you think that a P4 overclocked to 5.2 GHz is twice as fast as a stock G1610 running at 2.6 GHz all across the board? Or is there much more going on then one (relatively insignificant) spec? In fact, could it be argued that certain specs hit a plateau and previously irrelevant factors become bottlenecks? Intel certainly found this with latency of electrons in the core 2 era.
On topic of HDDs and platter density increases. One could take an overly simple article written over at Tom's Hardware
and read the first line
Increased data density typically results in better throughput
then call it a day, close the book, and go about life. Unfortunately they'd be missing a myriad of reasons that contributed much more
to those throughput differences than just the simple "data density." Maybe it fails to mention things like the fact that the newest drive (1TB platter) also included a DDR2 DRAM cache with improved caching algorithms. Or that the drive uses a built-in dual-core processor and secondary actuation. All of which improve throughput with the exception of actuator
It's good that virtually everyone who knows anything feels free to attribute all improvements to platter density. It was true back in the day of floppy disks, so it must still be the leading differentiator in disk throughput