If you're using JPG file format (the usual setting in most cameras) then the file size will not be the same as the number of pixels.

Many cameras you can change the quality of the file (use the highest) and sometimes change the image size (use the largest).

The important thing here is that the JPG file format uses compression to make the file smaller while keeping the pixels. Sort of. It is a lossy compression, so some of the details of the picture are thrown away to make the files smaller.
That results in blocky patches in the image where the colour is nearly the same (like in the sky or fields) or loss of tiny details.

Even if you use highest quality, and largest image size, you will not get all the information that the camera sees - unless you can use a RAW file format instead of JPG.