Items with a large mass and a small surface area tend to transfer their heat quickly from one end to the rest of the item, so that the temperature at the point of initial ignition quickly drops below the ignition point of the item and the fire goes out.

For example, the same wood. In theory, the ignition points are the same. But a small piece of wood will catch fire more easily than a large piece of wood, and shavings are more flammable. Paper is similar to shavings in that it has a small mass and a large surface area, which makes it less likely to dissipate heat and ignite easily.

When making a stove, add wood, then add a few small briquettes, then add briquettes to make it easier to ignite. This is the same reason. Smaller coals are more flammable than larger ones.

Of course, the different ignition points of various items are also a factor. Wood itself is more flammable than coal.

Wood itself is more flammable than coal.