That is so wrong on so many levels.
The reason you have to decrease the size in photoshop to make the photo sharper is because the camera is using aggressive noise reduction algorithms which do a good job of removing the noise but also removes details. When you resize the photo you aren't making it any "sharper". It just apears more sharp at the per pixel level. You aren't creating any more details. They are lost forever when the camera does the noise removal.
To get a sharper photo you want less noise reduction. To be able to use less noise reduction and get a photo with an acceptable amount of noise you want a sensor that is highly sensitive to light. The easiest way to do this is with large pixels. Large pixels are physically larger and therefore able to gather more photons (light) and less noise. Large pixels also mean fewer pixels if the sensor were the same size. This is why less is more in terms of pixels.
There is, as always, a but. If you compare the camera in the current Iphone which was launched in 2007. It is already very old. Newer camera sensors have better hardware which changes the analog signal each pixel generates into a digital signal (signal amplifier) and other hardware which removes noise even before it is turned into digital data. This is why an 8mp sensor has less noise than some 3mp sensors.
If you would make two camera phone-size sensors with the same technology, one with 3mp and one with 8mp then most likely the 3mp sensor will retain more detail than the 8mp one.
