Insulin by itelf is not responsible for weight gain. If there is a good balance between food (and caloric need) and insulin that was the insulin is doing is making it possible to use the food for energy. If a person is overconsuming, and doses insulin to cover that amount, then of course weight gain happens.
The weight gain that occurs upon initial diagnosis of type 1 and start up of insulin is do to the fact that the body isn't "consuming itself" to survive. Basically it is the excess food - food consumed in excess of need - that is stored. The same thing would happen with a non-diabetic person.