Burnout does look a lot better but the reason why is you have no where near as much stuff going on. All you have in Burnout is traffic. No pedestrians, cops, motorbikes, drivers in the cars etc. All that extra stuff going on has clearly had an impact on how much power is left for the graphics.
When you look at the early trailers the graphics looked a lot better than they do in the actual game, but then tweaking framerate is normally one of the jobs in the final stages of production. With the amount of time R* have put into the game and given that there was a 6 month delay, I'm sure they have got the graphics looking as good as they could given the scope of the game IMO.