I attempted to duplicate what you have done with CS6 to see if I replicate the problem. I used H.264 and used the same settings you did with different results. The rendered video is 1280 x 720. I checked it twice, once by checking the file properties and once using GSpot v2.70a. Since I am not a gamer I could not use fraps to capture video from a game to make the exact same comparison.
There are noticeable differences between the videos you posted on YouTube. The text, for example, is much easier to read in the clip rendered with CS5. Has anything changed hardware wise?