It's been over four years since the first iPhone came out, and a lot has changed, from the screen to the CPU to the form factor. How drastic has this change been over the years? Here's a look at the iPhone's evolution, from 1 to 4S.
Each line of the graph represents a different iPhone spec that applies to every generation. Each mark along the x-axis represents a new iPhone model. Every subsequent increase or decrease doesn't correspond to the raw numbers of each spec, but the factor by which it increased. Every spec in the original iPhone starts as a baseline value, hence the flatline to start.
So what's changed the most? Statistically speaking, the camera, CPU and the RAM have experienced the most drastic changes over the five generations of iPhone, with the camera being improved every generation after the iPhone 3G. Weight has stayed fairly consistent (+/- 1 oz), and while the decrease in thickness seems incremental, its a spec that's hard to quantify. In any case, wrap your brain around this chart and chew on those thoughts.