Considering you can only play the game if you are connected to PD's servers, I think it is safe to say that PD has all the data on every track in their catalogue. How many miles driven on each track from all players, how many hours raced on each track, etc. I'm sure the big 4 are the most raced, most miles driven, most hours spent tracks in the game. Now, PD could look at that and say, "Well, clearly those are the most popular because they pay the most". I would think, as a developer, you would want to see all your tracks getting similar race hours/miles spent, but I'm sure they can see that Broadbean Raceway gets very little, if any, play on it. They could increase the payout for all the other races, but even if they increase the other races by 100k/race, the big 4 are still the best ways of making money. Thats why I've recommended that every monthly update that PD rotates the big 4 payouts to different tracks. It keeps the game feeling a bit more fresh as you can grind all the different tracks, testing and tuning all sorts of different cars. But they will never do it.