My iPhone barely makes it through the day sometimes. I usually have no problem, but occasionally I find myself around 9PM and my battery is down to 5% and I have to plug it in somewhere. This leads me to plug it in more than I strictly need to, just to ward off these moments. This would all become a non-issue if the battery were twice the size. Then my phone might be 10mm thick instead of 7mm thick, and might weigh 300 grams instead of 200 grams. Not a big deal. But instead, thinness is maximized, and everything else is built to be just adequate based on that.