The interesting thing is that the iPhone and the iPad actually have the same battery life
if you use them for same things:
- Internet use/surfing the web on Wi-Fi: iPad up to 10 hours, iPhone up to 10 hours
- Internet use/surfing the web on cellular data network: iPad up to 9 hours, iPhone up to 8/10 hours (3G/LTE)
- Watching video: iPad up to 10 hours, iPhone up to 10 hours
- Listening to music: iPad up to 10 hours, iPhone up to 40 hours
- Talk time: iPhone up to 10 hours (on 3G)
- Standby time: iPhone up to 250 hours
The key difference is that most people actually use their iPhones more than their iPad. Firstly, I'd say the majority only uses the iPad when at home (+ while travelling), that already limits the time spent with the device compared to the iPhone which will be used all day long. Secondly, most people are sitting or lying down when using the iPad (exceptions would be an iPad streaming audio to loudspeakers), the iPhone is also used while standing or walking (in particular for listening to audio and talking).
Partially because we generally spend less time with our iPads, we also use them for a smaller number of tasks. Just think how many apps on the iPhone have you used in the last seven days and how many on your iPad? And because the iPad is a more stationary device, we also put less stress on the radio connections (more stable, less changes, on average better signal strength).
Being more stationary a device, we also are less bothered by weight and thickness with the iPad compared to the iPhone. If people weren't bothered by thickness and weight, all those not making it through a day on one charge would be happy using external battery cases.
Based on all this, it is reasonable to say that the sweet spot of customer satisfaction based on battery life and device thickness and weight corresponds to a longer real-life battery life of the iPad. That doesn't mean that the iPhone currently is at the sweet spot. But it is hard to accurately determine the precise location of the sweet spot.