As per Moore's law, every 18 months computing power doubles. I found a similar empirical formula on the form factor. Every 24 months the form factor decreases by 2 folds.
Here are some comparisons...
Canon Powershot A 70 camera (bought in Aug 2003) - uses 4 AA size batteries
Canon Powershot SD 300 (bought in Sep 2005) - uses Lithium ion battery
Both cameras roughly has the same set of features, but the form factor has decreased by a factor of 2. Even in this case I could say it decreased by a factor of 3 or 4.
Similarly the CF card
SanDisk 256 MB CF card - bought in Aug 2003
SanDisk 512 MD SD card - bought in Sep 2005
Both has form factor difference of similar calibre.
Apple iPod Mini
Apple iPod Nano
iPod nano has really flattened everything that Mini had. Nano even had a special flattened pin for USB connectivity.