The reason the Dark Side tends to appear stronger has two reasons. In the old trilogy, we first see Obi-Wan duel Vader. Obi-Wan loses (intentionally) but I would find it difficult to argue who has the upper hand. The more interesting are the other duels, which are always fought by Luke on the light side. Luke has a lot less training than Vader and is no match for the Emperor. This has purely skill reasons, he just never had the chance to practice lightsaber dueling and was regarding the Emperor, well he just didn't expect that.
So it looks like the light side is weaker because of a sampling error. We just don't see enough fights between Jedi and Dark Side practitioners.
This of course set the expectation of what a fight should look like and in an attempt to exploit the old trilogy for its nostalgia as much as possible, Lucas made the new films continue on the same line.