Numerous studies have shown that physical activity does not have a significant effect on weight. For example, a meta-analysis published in the Journal of the Academy of Nutrition and Dietetics which included over 1,000 people, found that when diet stayed the same, increasing workouts from less than 1.5 hours a week up to 3-5 hours a week, over the course of one full year, lead to an average weight loss of 3-4 pounds. You could healthfully lose that in as little as two weeks by changing your diet
Another meta-analysis published in PLOS One that examined the relationship between fat mass and physical activity in children found no association between the two, and concluded that physical activity may not be the key determinant of unhealthy weight gain in children.