It was for the first time in the film 'Marana Mrudangam'( 1988) 'Megastar' title appeared as a prefix to Chiranjeevi's name in the title cards. Till then, titles like Dynamic hero, Supreme hero were prefixed to Chiranjeevi's name in the the title cards. Finally, Megastar title perfectly suited to Chiranjeevi's stature. From then on, both Megastar and Chiranjeevi turned doublet and one could not separate and breach both the words, as it seems.
Although media addresses the top heroes of other languages as 'megastars' every now and then, the title got familiarity with Chiranjeevi only. And Google search engine also says that. When 'Megastar' word is typed in Google Search, it simply showcases the details of only Chiranjeevi, that in turn betokens how popular Chiranjeevi as a Megastar.
On the other hand, if the word 'Superstar' is typed, we get the definition of the title but no heroes are showcased by the Google. That's the difference between Megastar and Superstar.
