Thompson revealed that she almost quit 'Brideshead Revisited' because the film's producers asked another actor to lose weight.
Thompson made the comments during an interview on a Swedish talk show after another guest on the show brought up the issue of weight loss.
The Hollywood star refused to reveal the name of the actor who was asked to lose weight, but confirmed she confronted the producers over the issue.
"Actresses who are in their 30s simply don't eat," she said.
'Brideshead Revisited' was released in 2008 and starred Thompson, Ben Whishaw, and Hayley Atwell. The film was an adaptation of Evelyn Waugh's novel of the same name. Other female actors in the flick included Felicity Jones and Greta Scacchi.
During the interview, Thompson also joked that Hollywood's obsession with weight issues is the reason she chose not to live there.
The actor also criticized the fashion industry, especially the French one, which promised to get rid of 'size zero' but did not.
"There are some subjects you have to keep noises about because it's so tedious, and it's gone on and on," Thompson said.
Thompson has crafted a reputation as a keen proponent of human rights in the past. In a letter to British publication the Guardian last year, she urged London voters to support the Women's Equality Party in the city's mayoral election.
In the letter, Thompson said that her reason for backing the Women's Equality Party is because she does not want to die without closing the pay gap.