By taking a little less now, may payoff tons more in the future. By not crippling NE with the salary cap issues of a big contract, NE will be more competitive in the future. Which in turn ensures his enshrinement in the HOF.
Which will lead to many more valuable endorsement contracts after his football career is over.
If he took a greedy big fat contract now, the team may falter and lessen his career stats and make him more average and less bankable.
It's a small gamble on Brady's part to get paid tons more later on. Heck a 60 mil contract is not exactly going to leave him wondering how he can pay his bills. He is a pretty frugal guy to start with isn't he?