The Hollyoaks actress told New that the change has lasted despite the show ending.
"They got bigger during the show and now I'm like, 'Whoa!'," The Sun quotes her as saying.
"I don't know how it happened. My mum asked me if I'd had a boob job. I was a B, now I'm a C. I'm dead lucky."
Porter added: "The thing is now when I eat, I put weight on. I'm a frump version of what I was on the show."
The soap star has previously said that she would "prefer more curves" and claimed earlier this month that she does not believe she is attractive.