IRISH teen idols Westlife arrived in the States earlier this month and announced their intention to become the biggest transatlantic pop success story in years. But bosses at the boys' American label, Arista, have decided that the fresh-faced five some are quite big enough already and ordered them to slim down before they begin filming a new £300,000 video for their British number 1, swear it again, a week today. "They've hired a personal trainer on £7,000 a week to put the band on an intensive slimming regime," whispers our mole on the Westlife camp. "They're taking it very seriously and have canceled an appearance with Mariah Carey at next month's Brit Awards because they want to concentrate on America." A spokeswoman for the band, who are co-managed by Boyzone's Ronan Keating, tried to make excuses for the porky pop stars. She said "They are young boys who don't know how to eat properly. They do have a personal trainer who has been working with them. She helps them exercise as well as advising them about what to eat." Unfortunately band member Bryan McFadden seems unable to take her advise. "I can't stop eating McDonalds burgers," he admitted. And the rest of the band seem similarly undisciplined. "They have been offered traditional food but have said they would rather have chips and beans," said our source. "And while they're not rock and roll drinkers they do like their Guinness, which puts on weight."