It has to do with the number of colors and tonal range in each image, Roy. A black and white image will compress smaller than a color image, for example, even though both are the same dimensions. Your first image has fewer colors than the other three, so I'm betting that's the one in the low teens. And so on ...
it also depends how well it can compress. like an open sky or a flat wall will be really small in size. if it has to keep track of more data, it will be larger. it's one reason why high iso takes up more space than low.
Deborah - reverse order - the train was first the desert mounds last. I actually thought that was the case - about the color and tone - but it still makes keeping an image under 25 mg a "guessing game" as each is hardly touched, did add my signature, but otherwise the same size out of the camera. So there is no "real" formula for making an image a certain size.
Why do you upload a png?
1. It's larger, takes longer, and ironically, you can't upload as high a resolution high quality image.
2. FAA converts it to a JPG anyway. So if you convert to jpg yourself, your upload is smaller, and conversion is under YOUR control.