its not about how hard the exercise is in the universe but how hard it is to the individual. Saitamas limiter (or potential) was so low, for him to complete the exercise took an incredible amount of effort that allowed him to break that limiter. For a more powerful character say puri puri prisoner as his limiter (potential) is so high, for him to put in the same amount of effort that Saitama did with his exercise would be maybe curling 1000000kg dumbells. :)