Obesity is technically defined as a BMI of 30 or more. (Body Mass Index, calculated by dividing bodyweight in kilograms by height in meters squared; BMI = kgs/mxm. BMI calculators can be found online.)
Fat deposits around the abdomen are thought to be the main cause, putting chronic strain on the bladder and urethra, as well as stretching and weakening the nerves and muscles of the pelvic region.
There is a strong correlation between the risk of urinary incontinence and each additional BMI unit for people with obesity. Encouragingly, there is also a proven reduction in the incidence of urinary incontinence as the BMI reduces. So weight management should improve the issue and in some cases, even resolve it.