Wire sizes vary with the expected current they will carry. Larger wires (smaller gauge number) are needed when currents are higher.
Think of a river and its smaller tributaries, where the battery is equivalent to the a lake or reservoir. All the waterways leading to the lake stat small and get larger as they reach the reservoir. Similarly, a turn signal lamp's green wire only needs to carry the current the lamp uses. However, the green wire connection to the frame at the coils must carry all the current for all the devices that use power on the entire bike (except for the starter motor).
Using too small a wire causes extra voltage loss and even wire heating. Electrically speaking, too large a wire is better, except for the extra expense and weight they add.
Honda used metric wire sizes, which do not equate with American wire gauge size exactly. See:
http://www.simetric.co.uk/siwire_elect.htmSizing a wire to the load entails determining how much heating the wire has to withstand in order to not melt it's insulation. The same wire can withstand more heating when there is air circulating around it, than when it is wrapped up in a bundle. The wire heats with the power it carries/ vs. it inherent resistance. The larger the wire cross section, the less resistance it offers.
There are also different types of wire insulation. Some melt at a higher temperature than others. The better grades of wire with either have a marking along its length or have a spec sheet/label on the spool.
