A lot has changed with the iPhone over the past decade, but one aspect that has changed less than others is battery capacity.
While the processor and graphics chips in the latest iPhone 7 are 120 and 240 times faster than those found in the original iPhone, the battery powering this silicon isn't 120 times bigger. In fact, overall battery capacity is one of the things that has perhaps changed the least in the iPhone over the decade.
For the purposes of this piece I'm going to measure battery capacity in milliamp-hour (mAh), which is the energy charge that a battery holds. The bigger the number, the more power the battery contains. In theory a 2,000mAh battery should last twice as long as a 1,000mAh battery in the base device (in theory things are a little more complex, but for the purposes of what we are doing here, we can take this to be true).
Here are the battery capacities for the iPhone over the past decade (excluding the 5c, SE, and Plus models):
Taking the iPhone as the starting point, battery capacity has grown by only 40 percent. If we take the iPhone 3G as the starting point, that number jumps to 70 percent.
But, still, this means a massive increase in performance with less than a doubling of the battery capacity.
How has Apple pulled this off?
In a phrase - power efficiency. With each iteration of the iPhone, Apple has learned how to squeeze more performance out of every component. While the processor and graphic chips are the most obvious, Apple has made power efficiencies across the board, from the display to the radios to the speakers.
Even tiny efficiencies add up.
Another thing that Apple has leveraged is better battery chemistry. While the power coming out of the battery hasn't changed, better battery chemistries mean that Apple can get more power of of cells without making them bigger. In fact, one of the ways that Apple has been able to make the iPhone thinner and lighter over the years is by shrinking the battery.
What's interesting to note is how little battery life by usage type has changed over the decade, with talk time and internet usage staying pretty flat over the 10 years (there's an excellent chart here that shows the change).