System Information
Windows 10 64 bit, 980TI

Blender Version
Broken: Recent master (cb1cb63)

Short description of error
When saving a render to the OpenEXR format, all the codecs (except the first two[1]) produce files of exactly the same size. This makes me think that none of them are actually doing any compression.

I tested both RGB and RGBA, Float (Half) and Float (Full) - the Codec option still makes no difference.

[1] DWAB and DWAA seem to be the only codecs that produce different size files.

Exact steps for others to reproduce the error

Save any image (e.g. a render of the default cube) several times using the OpenEXR format, and set the Codec to something different each time.
Here's a zip of my own test: