Radlafx has pretty much nailed it.  The size of the JPG image file is determined by the amount of detail in the image...that's why you will note that not all your images out of a particular cam are the same size.
Noise will increase the file size, as will increasing the in-cam sharpening settings.  Also remember that the # of shots left is just the cams guess....the actual # will still vary depending upon the detail contained within those remaining shots.

I guess that really, saying there is more detail is not quite accurate....the cam picks up the same amount of info which is determined by the total # of pixels....but what happens is that compression is much more efficient with less differentiation of pixels.  That is why a JPG image of a plain blue wall will be much much smaller in size than a JPG image of grass...given the same JPG compression ratio.

So, if your cam does not compress RAW files (some do)...the RAW files "should" all be the same size....

In youth, we learn....with age, we understand.