In an interview with The Hollywood Reporter, the Black Swan star says: 'I definitely see skinny people as sad now. It's so sad to be skinny...
'I used to be like, 'Oh wow, I want to look like a model too'. And now I'm like, 'They're sad'.'
Portman, 29, makes her comments as part of a special roundtable interview with five other actresses - Amy Adams, Helena Bonham Carter, Nicole Kidman, Annette Bening and Hilary Swank.
Adams, 36, became a mother earlier this year. On the topic of weight, she says: 'I don't have the discipline to be skinny, like truly skinny.'
Although Portman says: 'For a few months, it's possible...'
Adams also hit back at critics of her figure. Asked what surprised her most about working in Hollywood, she says: 'I didn't know I'd have to be a model.
'I focus so much on the work and then all of a sudden I'm in front of a camera and they're picking my neck apart, and I'm like, 'I'm 5ft 4!'
'I'm not going to look tall; there's nothing we can really do about that.'