There's a possibility that the number of actual pixels shrinks due to a change in resolution. The x-y "size" of the image should not change. So I think the app you are using to "copy" the image is actually performing a "downsizing" function, which is common when editing a JPEG image with any software when the "save" option is set to less than 100% - and even then, some pixels are lost. This is why it is preferred to capture an image in RAW, edit in RAW, and be wary of copying a Canon CR2 image since that might result in converting it to some other format.
For what it's worth, the Apple computer does not alter the pixel content (resolution) of an image in a copy operation. It only changes the date/time stamp. This is also "normal" for Linux and Unix.
