Thinking something is sexist doesn't make it so, unfortunately for them. Note the definitions of sexism even with the biased language in favor of women (which I oh-so-loathe) it does not support their argument that the
design of the character is sexist.
http://www.merriam-webster.com/dictionary/sexism
1.) prejudice or discrimination based on sex; especially : discrimination against women
2.) behavior, conditions, or attitudes that foster stereotypes of social roles based on sex
One character having large breasts does not show any sort of prejudice or discrimination. Women have breasts, sometimes they are large. In this case, they are large. And if we want to talk about her attire, then you'd actually probably be angering many feminists everywhere. Look up the "Slut Walk." Many women and especially feminists actually enjoy dressing however they want. Your puritanical views be damned. There's nothing sexist about her attire.
Now, definition two is probably what many are latching on to but I don't see how the Sorceress's social role has anything to do with her design. She still weaves magic and still has just as much agency to get things done as any other character in the game. (Have you seen her walk animation? She's one confident girl to boot!)
She kicks ass and looks sexy while doing it. Why wouldn't a woman feel empowered by that?