The $15.925M signing bonus in the first year of Matthews' new deal will be the biggest bonus in NHL history. This does a couple of different things. It means Matthews pays less taxes on his money and should the Leafs decide to move on from Matthews at any point (they won't) because the deal is front loaded with most of the money in bonus and not in salary, it'll be a little easier to do so.
Not only does this deal set the market for the next crop of free agents, but and this will make a lot of people groan, Matthews is absolutely worth the money he got. Not only does he have the most goals of any player since he entered the league in 2016-17 with 299 (that's two more than Ovechkin) but he plays a premium position and he blocks shots like no other forward. Not to mention, Matthews had 85 points and 40 goes while admitting he played the entire year with a wrist injury. He's worth the money.