Go to ifixit.com and look at them.
Nothing fundamental has changed about battery capabilities/design between the 5 and 5s. The difference in capacity is almost entirely due to the difference in physical dimensions.
These things are like jigsaw puzzles - you can't change the size/shape of one puzzle piece without changing every puzzle piece surrounding it. You'll see that other key modules in the 5s have changed size and shape, ultimately making more space for a larger battery.