The Grammy winner explained that food has "controlled" him from the time he was a child.
"When I was at school and wasn't having a great time or when music wasn't going very well. I would eat, eat," he told 60 Minutes Australia. "Eating would make me feel better. When I felt lonely I would eat."
Smith, 22, said that he is more offended when people make comments about his weight than when people utter slurs about his sexuality.
"I think just because I've accepted that, if someone calls me a f-----, it's like, I am gay, and I'm proud to be gay, so there's no issues there," he said. "If someone calls you fat, it's like, that's something I want to change."
Last week, Smith, who has been working with nutritionist Amelia Freer, said he was just 4 pounds from reaching his goal weight.