'the most equal teams competing'
This is very likely in the higher levels, but it depends on the server too, and if most teams have say a quality of 6-7 stars, where it becomes very difficult to add more quality (unless you have a lot of offers or money to spend), the league becomes very tough, where each match may make a big difference. In my opinion, generally the significant difficulty for winning a league starts when a league's average quality is close to 100% and beyond in the first days of the season.