The 5 and 16 MP are jpg's and jpg always reduces the original camera sensor picture, with smart calculations, to a smaller size. So you will never get a 5 or 16MP file size picture. (that's basically the idea behind jpg formats).
If you want big size files you need to use DNG format.
As an example if you take a picture of a green door the algorithms are so smart that the size of the picture can be reduced because the color is everywhere the same and does not needs to be stored for every pixel, hence reducing the file size.
Now the camera sensor of the 1520 will use 16mp pixels and your phone will create a jpg. But if you use a camera with a 5 mp camera sensor than also this phone will create a jpg.
And the size if the pictures may be the same but with the 1520 you will get a better picture because you started with more "data".