The upper Xperia echelon is known for its slim design and the Z1 is no exception - 8.5mm doesn’t sound wafer thin, but not until you realize that inside this phone’s body there’s a 1/2.3" sensor and a 3, 000mAh battery. The sensor is 70% bigger in terms of surface area than what smartphones usually get(1/3") and large sensors require bigger optics, hence thicker phones. The same goes for high-capacity batteries.
Of course, the big, high-resolution sensor will give the Xperia Z1 a leg up on the competition in the imaging department. But Sony is also responding to Nokia’s propensity for name-dropping(ZEISS lenses) by using its own sensor brand and lens design, plus a BIONZ image processor.