It is all largely a function of when you become a free agent. Max contract sizes are tied to your experience in the league and the percentage of the cap you are allowed to occupy for your team (0-6 years - 25% of the cap; 7-9 years - 30% of the cap, 10+ years 25% of the cap). Essentially, Tatum and Brown signed the exact same contracts but Tatum's came a year later and therefor is written into a new salary cap ($136 mil last year, just under $141 mil this year).
As for who is next, Donovan Mitchell just signed a 3 year extension with the Cavs. He was eligible for a 4 year extension but chose to make it 3 years because that will allow him to become a free agent in 2027 when he will have 10 years of NBA experience. Thus his next deal will be for the 10+ year Max which is expected to be 5 years for $380+ million, $70+ million more than what Tatum signed for.
But the guy who will break the record first will be Luka Doncic. He is a free agent next summer and will be able to sign a 5 year deal for something close to $346 million.