Also Gone With the Wind was released three or four times in cinemas over a 20-year period I believe, so a hell of a lot of people would go and see it for nostalgic reasons or may not have been born the first time around. I think the first release only garnered a little over 20m ticket sales which was pretty disappointing, because of expensive tickets. A re-release a year later was much cheaper and garnered much more interest. Also it was very popular in places like London, and I assume that those figures must factor into the 200m (the global market had a lot more importance back then than it does now).
And also while the potential audience might be lower, the audience share that cinema had would be many times greater. No TV, no internet, the biggest competitor was probably the radio, and just about every family would go to the cinema once a week. There's a brilliant moment in Public Enemies when John Dillinger goes to a cinema just to escape the summer heat - cinemas were frequented an awful lot more back then. So really when you adjust for inflation it's almost inevitable that older films will come out on top. ET or Return of the Jedi were probably the last blockbusters released that didn't have to compete with cable television, so when you scan the inflated box office lists it makes the accomplishments of films like Titanic and Avatar even more impressive than those which came before.