I think creating graphics for a flight game would be much more difficult to achieve than a racing game. There is more to create, such as all types of geography, weather conditions and cityscapes. I haven't played AC 5 yet but I'm sure each level/battle ground is massive as you control fighter jet planes that cover large distances in minutes.
Gt 4 has highly detailed tracks and 100's of cars also detailed to an absolutly impresive degree, but I wouldn't think that GT 4 contains such varried, vast environments that AC 5 has.