How can I preview an image in its original size? I have 2 images in a directory and I see from the Get Info that one has Dimensions: 532 × 1051 and size 45KB on disk. The other based on the Get Info has Dimensions: 1440 × 2900 and size 379KB on disk.  

Problem

When I open them with Preview they both show in the same size although the resolution is better in the latter, but how can I preview them and have the images open in their original size? I think the one with 532 × 1051 should be smaller when opened.

I see that there is an option in Preview -&gt; View -&gt; Actual Size, I selected that but the size remained the same (slightly zoomed in) and now that option is grayed out.
"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").