[Delvo]

I once saw someone talking about "gray Jedi", who could use the emotional intensity of the Dark Side when needed but still calm down and maintain control when the need was past. That person seemed to think that this was an established idea, among some faction of the fans at least even if not in novel/comic canon. Mace Windu was suggested as an example (and his lightsaber was purple, which is half blue, half red!), and I think the concept is really the only explanation that makes any sense for Obi-Wan's last-minute defeat of Darth Maul soon after seeing his master killed.

And when Luke asked whether the Dark Side was more powerful, Yoda said it isn't, but it that it is "quicker, easier", so that's another possible explanation for the apparent imbalance: not that there's actually an imbalance of power, but that those on one side find it easier to use.

And Sith Lords could also have made a point of recruiting only the top Force-users, as opposed to all of them including the averages and below-averages like the Jedi Order did.