In both cases the media is the same size 22.5x81.5 inches. The actual dimensions of the image, by contrast are quite easy to determine, they are given in the image dictionary. The area covered is calculated using matrix algebra and can be a combination of several different matrices. That's a difficult thing to measure in a PDF file. So you can see that, in PDF, the effective resolution of the image is a combination of the dimensions of the image, and the dimensions of the media it covers. The effective resolution of the first image is 1000 dpi, the effective resolution of the second is 500 dpi. the first image is 1000x1000, the second is 500x500. Now consider I have two images drawn in 1 inch squares. The image hasn't changed, just the area it covers. If I change my mind and draw it in a square 4 inches by 4 inches, then the effective resolution is 250 dpi. So if I have an image which is 1000x1000 pixels, and I draw it in a 1 inch square, then the effective resolution of the image is 1000 dpi.
The effective resolution is given by dividing the dimension by the number of pixels in the image in that dimension. At this point there is no resolution, the pixels are laid down in a specific media size.
These are then drawn onto the underlying media. In PDF the image is simply a bitmap, a sequence of coloured pixels. The reason is that the images have a different effective resolution. In one case the image is ~48 MB and in the other its around 200 MB. However in your case both the files consist of nothing but an image. As fmw42 says PDF files themselves have no resolution.
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