Thank you, Mr. EPCOT, for asking which is "bigger", not which is "better". One has to admit that Star Wars is bigger, from a number of points of view.
However, the large gap between the first trilogy and the second trilogy allowed it to capture two, maybe three generations of devotees.
My dad took me and my brother to see SW, I enjoyed the heck out of it, but that was that for us. I didn't see Empire until its re-release, when I was dating my now-DH, who couldn't believe that I'd never seen it. We stood in line for hours to see Jedi the year we got married
eek
, and I could tell you a long story about the projector repeatedly breaking down and the near riot it caused, but this is not the time or place... Second trilogy, I thought they were less than stellar. DH claims to love them just as much as the first, but then he has bought the expanded universe comics and books, too, over the years, and I never bothered with any of them.
I read and loved the HP books. The movies, less so. I've just finished re-reading the entirety of the seven books. Extremely compelling writing. As soon as I finished Deathly Hallows, I wanted to go back to the beginning and read it again. I have urged DH to read the books, but he resists. I don't understand, since he has watched, and reportedly enjoyed, the movies. His loss.
All of that wordiness leads to this: SW has had the benefit of spanning generations, and having children introduced and encouraged to enjoy it by their parents, who had enjoyed SW themselves in their childhood. HP is huge with the generation that read them as children, as well as those adults who were open-minded enough to check them out. But Since there will likely be no more books the only way to bring the story to another generation, will be for those who enjoyed it in its prime to pass it along.
Personally, I prefer the HP series to SW, but the answer to the question is that SW is bigger.