Yoda never said the Light Side was stronger.
However, GL indicates on the AOTC DVD that the Dark Side seems stronger- it seems to give you more power and to achieve what you want (as Yoda says- Quicker, Easier, more seductive).
But this is a false view- not because it doesn't work, but (as Sadako mentioned) because the Dark Side ultimately leads nowhere.
Remember, GL's theme for the Prequel Films is symbiosis. In interview, he talks about symbiosis being the natural state of things (this tying into Balance). The Light Side is the epitome of this natural symbiosis.
In contrast, GL tells us, the Dark Side is parasitic-it feeds upon and destroys itself. We are given the example of the Sith- mighty and powerful and evil and all that... but they destryoed THEMSELVES.
There is no true power on the Dark Side. It LOOKS like power, but it leads nowhere, for eventually it destroys you. Only the Light Side can ultimately endure.
As far as kicking the crap out of things goes, though- the Dark Side has the edge. If it didn't have that edge, no-one would bother using it.
Here is GL commenting on the matter:
"The Emperor was trying to get rid of Vader, and Vader was trying to get rid of the Emperor. And that is the antithesis of a symbiotic relationship, in which if you do that, you become cancer, and you eventually kill the host, and everything dies."