I'm tired of the 'big & heavy' mantra. That's not why Toronto will fail to win, nor is it why they've failed to win in the past. Gritty, grinding, players are useful, so long as they also bring offensive, defensive, and transitional value to the team, and that is what those six teams have that balances their roster out...not some supposed physical edge. In fact, Toronto is technically a 'heavier' team that Pittsburgh & Boston, and is 11th in the entire NHL in that metric.
Toronto has been more physical in the past...even a few seasons back we featured Martin, Komarov, & Polak, all big hitters, and big bodies like Boyle & Fehr, as well as relatively physical players like Kadri, Zaitsev, & Borgman...none of that helped us win in the playoffs. In fact, the 2018 Maple Leafs were the 4th heaviest team in the NHL...Boston was 22nd.
Yes, St. Louis was a heavy team, and Washington (to a lesser extent) was as well. But Pittsburgh...heck no! They were 29th heaviest in 2017, and 23rd in 2016 when they won their Stanley Cups. Physicality was not a major part of their success...it was speed, speed, speed, and skill, skill, skill, and goaltending that won them those back to back Cups. Crosby, Malkin, Kessel, Bonino, Kunitz, Guentzel, Hagelin, Rust, Sheary, Cullen....Hornqvist was really the only 'big & heavy' they had among their forwards, and the defense was not exceptionally big either, featuring Letang, Schultz, Daley, & Dumoulin...only Cole was really a big hitter on that team's defense.
All I'm saying is that playing heavy hockey is a good thing, in the sense you need to be hard on the puck, win battles down low, and play with intensity and effort. Toronto's stars like Marner, Matthews, & Nylander need to improve in that area. But Toronto has guys who do great at that, just like every other team, in their bottom & top six...Mikheyev, Moore, Hyman, Tavares, Kerfoot, Johnsson. Big hits have little correlation to possession, let alone goals, let alone wins. Toronto's bigger concern should be team defense, having better structure, situational awareness, and playing as a cohesive, team unit, not individual parts. Also, they need Freddie to be big in the playoffs, like Binnington, Holtby, & Murray have all been for their teams. That is a much bigger factor than 'heaviness'.
tl;dr
Heaviness is a factor, and not totally unimportant...but it is not the major factor behind why the Leafs haven't won, and these teams have. For example, Pittsburgh has the most Cups of these teams in recent memory, and was by no means a 'heavy' team. What made these teams successful, beyond their skilled offensive cores, was incredible goaltending, attention to detail & defense throughout the lineup, good situational coaching, things the Leafs have not had Also, the Leafs have come up against Washington, Boston, Boston respectively in the playoffs, without home ice advantage, and pushed each series to 6+ games, all very close. One more win against Boston, and they could've been in the Cup finals easily.
