Sudden weight gain is defined as gaining two or more pounds in one week. The common reasons for excess weight are either eating too much food or exercising too little. This is not always the case, there are other conditions that can either temporally contribute to weight gain or to slowly add on the pounds.
People come in all sizes and shapes and some are prone to gaining weight easier than others. Some people are genetically pre-programmed to be heavier and larger in size and genetics may contribute up to 60% to body size and weight, a larger frame or an endomorphic body type which translates into a fuller, rounder figure.
Another reason is that some people have a slower metabolism and therefore gain weight easily but there are other conditions that can increase your weight which are not diet related and need to be addressed.