A week ago, I wrote about how both the Los Angeles Dodgers and Houston Astros are in rare historical company this season. According to FiveThirtyEight’s Elo power ratings — which measure a team’s strength at any moment — each is playing roughly as well as the fabled 1927 Yankees played. But this season’s top-heaviness extends well beyond just the Astros and Dodgers. Each member of MLB’s ruling class this season is unusually strong, which suggests that, come October, we may be watching the the most stacked playoff fields in memory. That’s great news for fans — but it’s also really bad news for the wannabes and would-be Cinderellas that are currently chasing the front-runners.One easy way to visualize the power balance of a league is to look at how its teams at any given ranking slot measure up to those from other seasons in the past. For example, the Dodgers have the best Elo rating (1602) of any top-ranked team through July 20 of a season in the expansion era (since 1961). Likewise, the Astros are by far the best second-ranked Elo team of the expansion era.Go down the line, and each of Elo’s top six teams carries one of the strongest ratings in modern history for its slot. The third ranked Washington Nationals, for instance, are more like the top team in an average season than a mere third wheel. The Boston Red Sox would be running a strong third most seasons; this year, they’re a distant fourth. The Indians and Cubs can both tell similar stories. As we approach the July 31 trade deadline, this is more than just an academic curiosity. A team’s willingness to pony up prospects for a better shot at the World Series is directly tied to how much good it thinks a trade will do. In a wide-open season, even teams outside the top tier of contenders could be convinced to roll the dice on an upgrade — particularly with the expanded wild-card format. But the stronger the top teams are, the less incentive teams on the periphery have to make a championship push. According to Elo, we haven’t seen a stronger crop of elite teams in the expansion era than this season’s top six.1The top six teams of 2017 carry an average Elo rating of 1562.5, which beats out 1998 (1557.9) for the highest average for the top six teams through July 20 of a season since 1961.As recently as a few years ago, you could have lamented the lack of dominant teams at the top of the major leagues. At this same time in 2015, for instance, the leading Elo teams were among the weakest at their slots in the expansion era. But baseball’s era of parity seems to be officially over, with the game moving back toward imbalance. While a top-heavy MLB might never look like its basketball equivalent,2Even this season, the Dodgers’ winning percentage works out to a mere 57-win pace in the NBA — which would have only bought them the No. 3 seed in the Western Conference. it’s still going to be tougher than usual for aspiring contenders to break through — a fact you can bet every GM is keenly aware of in the lead-up to the deadline.