In a campfire, carbon from wood reacts with oxygen in a combustion reaction. If you've ever started a campfire, you know you must begin by lighting small pieces of wood (called kindling) instead of logs. Likewise, as this video shows, iron filings burn in the presence of air to make iron oxide. Why do iron filings burn while a heavy iron pipe does not? Why do twigs ignite better than logs? Give your answer in terms of atoms.

Increased surface area means more places where wood can turn into flame - where heat, fuel, and oxygen are all in the same place at the same time. The mass of a small particle is such that the entire particle can be heated to ignition temperature. An iron rod will not burn as it has relatively very less surface area per unit mass but iron filings with more surface area per unit mass can burn in air and get converted to iron oxide. Kindling or smaller pieces or wood have a lot of surface area and therefore have more contact with oxygen, making them easier to burn.

One thing I can tell is the fact that more surface area means more wood to burn into flames. When it starts to burn Heat, fuel, and oxygen occur in the same place at the same time to put it into flames.