A superheroine without big breasts is like a superhero without muscles. Besides we are all taught at a very young age that "sex sells". Why else would our children be subjected to subliminal messages in commercials like the bikini clad babe who thinks a guy is hot because of his choice of gum,deodorant,after-shave,pizza or whatever you can think of?
If a superheroine wasn't "busty" with a nice ass and tight outfit they'd be Richard Simmons. :)