Hi Photo Club I've just stumbled across your post whilst looking for something else but I might be able to clear this up for you. Sorry if I'm telling you something you already know!
The file size of your photos - 3mb, 5mb or whatever - has nothing to do with the resolution (20mp, 64mp etc) of the camera and everything to do with whether you are shooting jpegs - which the majority of smart phone photographers do - or RAW files.
For ease let's say you have a 24mp camera which means the image sensor is 4000 x 6000 pixels. But you will never get an equivalent file size out of it unless you shoot RAW format when the camera will save the totally uncompressed and unprocessed 4000 pixels x 6000 pixels to storage. You will then get a huge file size, maybe 40+Mb because you are storing every last bit of image data.
In the majority of cases people shoot jpegs where, in this example, you will still get a photo resolution of 4000x6000 (check the image data to see) BUT the file size is entirely dependent on how the compression algorithm deals with the contents of the image. If the image has lots of areas of similar colour the compression algorithm can store this in less space than an image with lots of areas of different colours. So you will still end up with a relatively small file size related to the efficiency of jpeg compression and NOT the number of megapixels in the sensor.
The number of megapixels is only significant if you want to resolve tons of detail and crop your photos significantly. Don't confuse megapixels with file size or image quality.
If you want the cynical view camera manufacturers use megapixels as a sales ploy (mines bigger than yours...) but the camera then throws away 95% of the tonal and colour information captured to create a conveniently small file size.
Hence you can buy ever higher resolution sensors but your final file size won't change that much.
