Why is megapixels a measuring stick at all then?
Because it's an easy metric for laypeople to comprehend. It's very easy to market a device with a 20 MP sensor and say that it's better than a 12 MP sensor. Anyone can market "bigger is better".
Ironically, in sensor size, bigger is truly better. The problem is how sensors are measured. You'll have a sensor measured at 1/1.7" compared to a 1/2.3" and most people (who suck at math) will think the 1/2.3" sensor is bigger, when it's the exact opposite.
In typical use, once you get passed around 6 MP, you're not gonna notice too much difference. The exceptions are the MP are useful if you crop your photos, print them out at 16x20 sizes, or view them on very very high resolution screens.
Otherwise, you're wasting storage space (more MP means more MBs).
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The bigger the lenses the more light is let in right?
Lenses are tied to the sensor. I guess (I'm no physicist) the larger the sensor, the larger the lens, but the distance between the lens and sensor needs to be factored, and then you get into the focal length (could be wrong on the terminology) of the lens (i.e. wide-angle, tele-photo, etc). In general, lenses are rated by size. They're rated by stuff like distortion and other crap.
In a phone camera, it's irrelevant as these lenses are so damn small to begin with and the photos are such garbage compared to what a dSLR can produce. That's why software is so important to camera phones. It allows you to take photos that you could not otherwise capture on such a small camera.
The next evolution of the dSLR might be to combine the superior optics and sensors of the dSLR with the superior computional power of a phone. That could unleash all sorts of incredible images ... but then we're on a slippery slope of "what's real and what's not".