300EE320 is right. This is the type of misconception I try to clarify everyday (I teach physics at a college).
The diameter of a hole will always get bigger if heated for materials that have normal thermal expansion properties (99% of the materials have normal thermal expansion properties).
Think of this way.
If you have a shaft and a bearing and the shaft won't fit into the bearing hole (by a little bit), what do you do? You heat up the bearing so the hole is large and therefore the shaft can go through.
If the plug (steel) and the hole (Aluminum) are a tight fit when cold, heating both will make it easier to remove the plug because aluminum expands more than steel (twice as much).