It is not that the Dark Side is more powerful, it is that the Dark Side encourages one to use power. Yes, the Jedi are powerful but they never brandish that power. They are reserved and contemplative and are satisfied with that way of life.
Sith and Dark/Fallen Jedi want to brandish that power. They want all who can see and hear them that they have power others lack. Further, they always want more power than they have and will seek out any opportunity to do so....even if that power is fleeting.
Don't the Jedi brandish that power to fight back the Sith? Same exact thing.