But power and strength are two different things. When Luke asks Yoda "is the dark side stronger?" Yoda responds emphatically "No." He avoids the mention of power, however. The dark side is quick and easy, because it gives immense power to a jedi without proper discipline or control.

The good side of the force remains stronger because a sith lord is not able to control his power as well as a jedi.

To put it simply, if a jedi were cloned at birth, and the two were identical in every physiological and mental way, the jedi using the dark side would be more powerful, but not necessarily win because of his inability to control his power.

This is ultimately why Anakin loses to Obi-wan despite being immensely more powerful. His arrogance and recklessness lead to his demise. Obi-wan is the picture of control in his battle with Anakin, careful and clever. Using the force as an ally not as a tool.

This is also why Luke defeats the Emperor. Luke refuses to give into the hate and recklessness of the dark side. In so doing, he becomes victim to the Emperor's attacks, but allows his father the opportunity to defeat the Emperor himself. At the same time, the Emperor's arrogance proves to be his demise as well. He believed that Vader was so far gone that he would not turn against him, he didn't think the rebels could successfully lower the force shield, and he didn't think they could blow up the death star. The Emperor is arguably the most powerful character in the original trilogy (who knows what Anakin would have done if he hadn't become half-robot), but he is defeated because he cannot control that power.

It is the nature of the dark side to pervert and destroy itself. Even the use of force lightning dramatically ages Palpatine.

In conclusion, sith lords don't seem stronger (in the force), they just seem more powerful.