The petroleum jelly is a fuel. The peroleum jelly is melted by the flame and wicked to the surface by the cotton. The petroleum jelly burns off first. After all the fuel is used up, the cotton ball burns up, but hopefully by this time your fire is well underway.

Technically speaking, the petroleum jelly does not make the cotton burn longer. A cotton ball will burn up in about 30 seconds. The petroleum jelly on the cotton ball will just burn for 2 or 3 minutes before the cotton ball starts to burn.

So you want the petroleum jelly on the outside and dry cotton on the inside because the dry cotton will readily accept the spark and turn it into flame which in turn starts to melt the petroleum jelly and bring it to the surface. This said, more petroleum jelly might be good as long as the middle of the cotton remains dry and you can pull the cotton ball apart without pressing all that extra jelly into the middle. The petroleum jelly probably also keeps the inside of the cotton ball dry too, so even though it might be a stretch, it does in effect water proof the cotton.

Cody Lundin even points out that the excess petroleum jelly can be used on dry skin. This makes your fire starter a multi use item.