If that is a dry food, despite its claims, it is more likely to put weight on than wet food.
If it is a gradual change to mostly wet food may help but if its wet, noooooooooo bloomin idea!
My cat Misa is about 6.5 kgs and Sasa is about 6kgs and they dont really eat a lot, Misa is a big cat, Sasa shouldnt be so big