How can I preview an image in its original size?
"Resolution" is not the number of pixels, but the number of pixels per inch, or centimeter, or other unit.

Most image formats contain data about the resolution: i.e. how many image pixels should appear in a given screen area. If the smaller image has a resolution of 72 pixels per inch, and the larger image a resolution of c. 200 ppi, then they will appear the same size on screen in Preview.

You can see the DPI setting for an image in Preview by showing the Inspector window -- Command I. If the image file has a resolution set, it will be listed there as DPI Height and DPI Width.

Preview does have a Preference to define 100% scale as either 1 image pixel = 1 screen pixel, or to use the resolution data in the image ("size on printout").