It would seem resampling in Image Size in Photoshop CC and Photoshop CC 2014 produces bad results under certain conditions (if the measurements are in inches and at least one side is below 1 inch apparently). Please see this thread for details: https://forums.adobe.com/inbox?object...
At first I could not reproduce this, but it turns out I was typing just 12 over the 6.
If I select the entire 600 first, then type 1200, boom, the problem occurs.
The root cause may be that while you're doing a specific sequence where the result has to go through 1 pixel/inch, some part of the dialog's logic must be working to ensure the image has at least a minimum 1 pixel x 1 pixel dimension. In doing so it loses the original size AND overrides the image aspect ratio.
I suggest moving the corrective "minimum size 1 pixel x 1 pixel" logic to the code that executes after the OK button is pressed.
It's the same for me...
For small images less than 1 inch, resampling redefines systematically width and height value to 1 (even if your image was a rectangle, it becomes a scare!!!!). This is a big issue! Resampling should not affect at all the real size of the image at any conditions!
This issue can be avoided by using pixels in the units dropdown menu for width and height. When using pixels as units, it works fine, but it bugs when centimetres or inches are selected.