The main reason for this is that Sunny Deol's films had instant appeal for a mass audience - they dealt with themes such as social justice and revenge that resonated with the general majority. Many of his films like Ghayal, Damini, Jeet, Ziddi, Border, and Ghatak also had deeper emotional nuances that helped him to connect with the class audience as well due to their tight screenplays and a bravura performance by Mr. Deol himself. Another reason for the huge openings are that the Deol family, as known to all, has a phenomenal fan following in Punjab, where a great chunk of their box-office success comes from.
Aamir Khan's films, on the other hand, were a bit unconventional and thus depended more on word of mouth to succeed. One must give him a pat on the back for going out on a limb and choosing out-of-the-box scripts like Jo Jeeta Wohi Sikandar, Hum Hain Rahi Pyar Ke, Andaz Apna Apna, Rangeela, Akele Hum Akeke Tum, Ghulam, and Sarfarosh that didn't really have mass appeal. All of the above films had Khan at his best performance with impeccable screenplays. While his films did roaring business in cities with a more educated audience that was able to appreciate the subtleties these films offered, the mass audience would be divided on whether to accept the film or not. Nowadays, the nation as a whole is more open to unconventional films, which allows even a film of his like Taare Zameen Par, which had him in a relatively small role, to be declared a hit.