"I've been a model and actress since I was 14, so I've been on a diet my whole life," the actress, 29, says in the March issue of Glamour.
"But once I gave birth, I didn't want to think about myself or feel insecure about my career; I wanted to think about my child. So I stopped worrying about diets."
Still, the mom of 2-year-old Milo (with husband Royston Langdon, 34) says she recently lost 10 pounds without even trying while filming the thriller The Strangers in South Carolina.
"I've been eating like a pig. I think it's because I hired this chef on the set of Strangers to cook for me, and I ate a lot of vegetables and protein. Filming was also incredibly intense: We were supposed to be out of breath in every scene, so I would run around the stage to achieve that. I just went to Marc Jacobs and I bought a red dress in a size 4. I've never been a size 4 in my entire life!"
She also weighs in on the debate over super-thin actresses and models. "I definitely think the girls look too skinny now," she says. "I'm friends with Helena Christensen and Linda Evangelista, and I remember Linda telling me that when she was a model [in the '90s], a sample size was a 6 or an 8. Now a sample dress size is a 0 or a 2. That's pretty alarming. There's a lot of pressure on [the models]. It's not healthy. I can't even imagine what that's like."
Tyler says she's more interested in being herself – and to that end, she took a two-year hiatus from film work so she could devote time to being a mom, and experience she describes as "transformational."