Superstar is a word associated with an actor when he has a huge fanbase and those fans come in hoards to watch the movie that particular actor stars in it.
I have always said this that Amir does not have that fanbase and he's not comparable to Salman Khan or even SRK in superstardom. BUT if you are one of the few fans (As compared to Salman/SRK) then you should be a proud fan as he has many movies that are genuine pieces of fine art.
You should never be indulging into arguments as who's bigger than who in superstardom because thats not your field of play. You should be arguing with an actor's fan who has great movies.
Salman and SRK have fans. I will give you an example:
Currently as they are in their prime. Even SRK as his "fans" will watch the movie no matter how crap it looks. So we have a JHMS which was a crap beyond imagination and only die hard srkians would have watched it and it had some 60-70 lakh footfall. Now we'll talk about Salman. He had Jai Ho(Not giving example of Race3 as it had a holiday release) and I don't really think people other than his fans would have gave that movie a chance as it was a low budget mediocre lookin movie. So the fans showed up and it had some 1 cr footfalls. You can even take Tubelight, the second most bashed movie of Salman and it also had more than 1 cr footfalls. Now we come to Amir Khan. He never really has come outside of holidays or to be precise Xmas but when he came it was Talaash and SS. Whereas Talaash was an appreciated movie and even I liked it. It ran on its merit and still it had 1 cr footfall and mind you it was an appreciated film. And about SS, everyone knows how amir promoted this movie and had it been Salman at Amir's place, we Salman fans would have watched the movie. SO SS had some 60 lakh ff which was also an appreciated movie.
To sum it up,
SRK at his all time worst: 60 lakh FF
Salman at his worst: 1 cr FF
Amir at his NOT worst/with appreciation from target audience: 60 lakh
You can see for yourself