Version 2.76b (Linux 64bit)
How to reproduce:
Open the attached file (just a monkey), and inspect the OpenEXR output settings.
Render with animation=True (ctrl+F12) so that Blender automatically outputs the one frame.
Render the same frame again and save the OpenEXR image with F3 using exactly the same output settings as before.
Inspect the two frame files and notice that one is much bigger than the other.
Both files should be identical, right?
I don't have the expertise to figure out the precise difference between these two outputs, but it seems that one of two things is happening: either Blender is outputting an OpenEXR image with more data than is needed when rendering an animation sequence, or worse, some data is lost when the image is saved with F3. In either case, there must be some parameter (color depth, compression, etc.) that is not being obeyed.
Please help me figure out what's going on here!