Brunel worked out his 7 foot gauge as the best, based on the science available at the time. The now standard gauge was inherited from a tramway, which is often said to be the width of a cart, or Roman chariot.
Of course the british polititions of the day decided that all track had to be to one gauge and chose the narrowet, because more of it existed.
PS I have read that the GWR didn't do its case any favours, because its locos weren't the best, and didn't exploit their gauge advantage.
Chris.