A person or thing that makes something swell or become larger.
A sweller can be something that makes things bigger or more full. It can be a good thing or a bad thing, depending on the situation.
Examples:
The medicine is a sweller that makes the bump on my head go away.
The chef is a sweller because he makes the bread rise.
The balloon is a sweller because it makes the balloon bigger.
The water is a sweller and makes the river get bigger.
The magic potion is a sweller that makes things grow huge!
The sweller is like a wizard that makes things get bigger.