OP, go and get yourself properly educated about your condition.
Is there an asthma nurse at your GP practice?
Can you have a good read of some national asthma websites?
You sound dangerously ill-informed about what the meds actually do.
Preventer = inhaled steroid. Is absorbed into the systemic bloodstream in minute quantities so will not have an effect on your weight. Oral steroids such as prednisolone, which you will have to take as rescure treatment if you don't take your inhalers properly will . They must be taken every day regardless of any/no symptoms to dampen down inflammation in the airways. They have no immediate effect on a tight chest.
Relievers, eg Ventolin, Bricanyl. Have an immediate effect on a tight chest. Must be taken if you feel any symptoms.
Being overweight is likely to make your asthma worse, not the other way around.