The group is determined not to be outdone by Take That, who have achieved huge success since their 2006 comeback.
Band member Mikey Graham said: "Yes, some of us have put on a few pounds over the past eight years. We know what we're up against with the likes of Take That back touring.
"But we're going to be trim and slim and we want to be bigger and better than before. I've been with the lads this afternoon and everything is going great."
However, he admitted that their image was in need of a revamp, saying: "Of course things are different - it's slowly morphing into a new thing as we're men now, not boys."
Bale embarked on a gruelling fitness regime to prepare for his leading role in 2005 film Batman Begins.
Ronan Keating recently announced that he had given up alcohol to prepare for the tour.