[melev]

Right. And thinner wire gets hot over time, which is why we use thicker wire for higher amperage usage.

If you had a licensed electrician do the work, he should know what is safe and what is not. He should have verified the thickness of the wire before replacing the breaker, or told you the wiring wasn't sufficient for a higher amperage breaker and thus would need a new circuit added. As long as he's a good electrician, there's nothing to worry about. If he's a new one or a journeyman, it would be nice to have his work verified by a certified electrician.