The Take That singers are both trying to shift the pounds after piling on weight since Christmas and Robbie is determined to lose more than his bandmate after learning he is starting a new eating plan.
The `Angels` singer told The Sun newspaper: "Me and Gary vary in weight. We go up and down by the week.
"Gary and the boys were at my house in Los Angeles last week when Gary turns to me and says, `I won`t be needing these clothes soon, do you want them?` - because he`s going on a diet.
"That was the day I thought, `Right, if you`re eating a lettuce, I`m eating half a lettuce.- "
The 37-year-old star has joked his weight loss ambition is to match Christian Bale`s size in `The Machinist` - a role which saw the British actor lose 60lbs by eating just a can of tuna and an apple a day.
He quipped: "There is now a serious game on to see who can get the thinnest for the tour.
"You know `The Machinist` with Christian Bale? I`m going for Christian Bale."
Despite Robbie`s joke, the `Kidz` hitmakers actually intend to get in shape healthily, with both of them working out with their own personal trainers.
They are also sharing a nutritionist called Jasmine who will work with them every day until Take That`s `Progress Live` tour starts in May.