No, all P&S do NOT have the same sensor size. Some, like the A6** serie have a 1/1,8" diagonal sensor (except the A650 which has a 1/1,7", a tad bigger). The same sensor size is used in the G serie. But the A5** and A7** use a smaller sensor, i.e. a 1/2,5" diagonal. Indeed, it may play a role on noise reduction. But you must also take into account the number of pixels squeezed on that small surface. In fact, what defines the signal/noise ratio (to make a long story short) is the ratio between number of pixels and sensor surface. And when the signal/noise ratio is low, the anti-noise processing has to make some compromises: keep noise and details, or reduce noise...and details. And I must say that Canon is quite good at that game, compared to other brands!