"It's not that unusual for actors to alter their appearance to play a part. But I think if I did it, it wouldn't get anywhere near the amount of attention," he told the Boston Herald on Sunday.
"It's absolutely fascinating to people that a woman would dare to do that — particularly someone who's very attractive and has a Hollywood-based career. They must sort of see her as reckless or something."
Zellweger, who stars this week in "Bridget Jones: The Edge of Reason," once again packed on the pounds to play the British heroine she originated in 2001 in "Bridget Jones's Diary."
"The sheer level of fascination on the subject is really a symptom of how this issue affects people, particularly women who are in utter disbelief that anyone would consciously go the other way — to actually try to gain weight — is mind-blowing," said Firth, who reprises his role as Bridget's lawyer-boyfriend, Mark Darcy.