Zellweger appeared fresh-faced atElle magazine's 21st annual Women In Hollywood Awards ceremony last month, sparking many to insist the actress had undergone major surgery.
Witherspoon has now addressed the criticism, saying it bothered her immensely.
"It's horrible. It's cruel and rude and disrespectful," she told The Hollywood Reporter.
"Why do they have to tear women down? And why do we have to tear other women down to build another woman up? It drives me crazy. Like, this one looks great without her makeup but that one doesn't look good without her makeup, and it's all just a judgment and assault.
"Men are prey to it as well. I just don't think it's with the same sort of ferocity," she added.