Rob, here are 2 pics taken side-by-side on my two 22" monitors. One is taken running the client through keyclone with maximiser set to 1680x1050 and the other is taken from running the wow.exe. As you can see, both have ui scaling turned on and to the lowest sizing yet the one running from wow.exe has more real estate on either edge of the action bars and the map seems smaller too so I'm not sure what the issue is.

Keyclone Client



WoW Client


[EDIT]
Hmmm, just noticed with them like this that even the UI options panel is smaller in the WoW.exe client