As has already been said, a low restriction exhaust will really only provide benefit at high RPMs, which is basically irrelevant to FE. A simple explanation would be that under light loads the engine does not have to push very much exhaust gas out the exhaust, so whatever restriction exists in the exhaust is likely not hurting efficiency, while under heavy throttle at high RPMs there is much more exhaust gas being produced, and a low restriction exhaust means the engine will not have to work as hard to push all this exhaust gas out. There are other factors that come into play but that is the simplest explanation. This is why I have left my Escort's exhaust unchanged, but have added a very low restriction muffler to my Firebird. Doing this produced no noticeable change in FE but did improve power. Plus it sounds cool.
I have heard it suggested that running an exhaust with too large a diameter for a given application will actually allow atmospheric pressure to work against the engine. I have no idea whether this is true or not.