Usually women with large chests don't have muscles.
Exactly. That's why most female comic book characters don't translate well to the screen..... Not too many ballerinas or brick houses (the 2 superheroine body types) have a F or G cups in real life so casting those parts becomes difficult.
I think sex appeal and believably is more important than just a large set of coconuts.
Considering comic books are generally not a realistic representation of the human form, it's a pretty silly argument. Admittedly, it's usually more extreme with female characters, but the men who have played comic book movie roles aren't exactly physical matches to the drawings of their characters, either.
If someone wants the actors in comic book movies to be near matches to the drawn proportions of the characters, they probably shouldn't be watching comic book based movies.
Good acting is much more important than close physical resemblance. Michael Keaton was a good Batman despite his lack of physical prowess. Heath Ledger looked very little like any of the comic book versions of the Joker I can recall, yet he deservedly won an Oscar for his performance. Hugh Jackman is 6'2" and plays Wolverine, who is generally described as little more than 5' tall, yet he has become synonymous with the character to many. I'll take a good performance over a badly acted physical match any day.