I was confused with same issue. For me this happened when an existing multimedia component is opened (the result was as in image 1) and when you create a new multimedia component (the result was as in image 2). But as long as you upload the correct image, it does not matters.
I tested this by clicking the image (the images are clickable as links in both cases). This opened a new window (in Chrome browser) and in the title of the browser you can check the image size. They were same in both the cases.
Tridion saves the images for preview at location: (Tridion_Installation_Path)\web\Preview
and displays the "toned down" image both in Preview and in preview of a multimedia component. The image of this URL is defined as:
http://tridion-doamin/WebUI/Editors/CME/icon.png?target=view&maxwidth=320&maxheight=200&uri=tcm%3A111-11111&mode=thumb&modified=2014-07-22T12:30:45
So the height and width is set automatically set to 200px and 320px for all existing images.
More detail here