System Information
OSX 10.10.3 but I guess irrelevant

Blender Version
Broken: 2.74.2 (3bb7e14)
Also checked RC2 and shows same 'bug'

in Video Sequence Editor preview screen, with view settings on Proxy size anything smaller than 100%
the preview scales down as soon as 'Show Overexposed' value > 0

Exact steps for others to reproduce the error
Start new file,
goto Video Editing Screen Layout,
open an image or movie so it shows something in the preview window
open properties with N if not visible
Set Proxy Render size to 25% (as an example)
increase 'Show Overexposed' ( > 0)
observe scaling of preview..

thanks for looking into it..
Best Regards,
Roel
IRC: kostex