* Almost a quarter of cancers in America are obesity-linked * High BMI linked to bowel, kidney, breast and other cancers * Proportion of cancers due to overweight is higher in women * Global problem likely to grow with economic developement By Kate Kelland LONDON, Nov 26 (Reuters) - Some half a million cases of cancer a year are due to people being overweight or obese, and the problem is particularly acute in North America, the World Health Organization's cancer research agency said on Wednesday.