Ted Kennedy had the good fortune of a long legacy by working in congress his whole career as apposed to running for the Presidency. Many laws were passed in the 40 some years he spent in congress that helped many Americans. It is unfortunate that his two brothers suffered such fates.
We, of course, cannot know for sure what sort of impact Ted Kennedy might have had on the nation if he had run for president. I'm not superstitious about things like this, but it was his decision not to run. There is no reason to focus on hypothetical situations; we should value any politician for the work they have done, not the work they might have done.
While Ted Kennedy did have a nice run in office as a sitting congressman, he was just one of many. As president, Ted would have cemented the Kennedy family as the most powerful in the land (during the time). However, I think that he just knew that he couldn't live up to JFK (or even Bobby), so he stayed in congress where he could forge his own way.
No, Ted Kennedy's legacy in Congress was not bigger than it would have been in the White House, however, it is understandable that he did not run for president when one considers what happened to his brothers. People do not remember Ted Kennedy in Congress in the way that they would have if he had made it into the White House.