You get the same amount of POWER from a given cross-section of wire in the mag field.
With smaller wire you can use more turns, getting a higher generated voltage - but a higher resistance, thus resulting in a proportionally lower current for a given amount of loss to heating.
Unfortunately, in a charging application the LOAD voltage is fixed by the battery. So you have to pick a good trade.
- Wind too many turns of a thin wire and you end up with low charging current, lower power to the battery at given wind speeds, and lower power to the battery at the stator-heating limit (where you must furl to prevent burnout). Go even farther and you may also start loading the prop at too low a speed, causing it to stall and keeping it from spinning up to where it would be collecting a lot more power from a given wind speed.
- Wind too few turns of a thicker wire and though you can get a lot of charging current in high winds your cutin speed rises enough to make you to miss power in lower winds when you need it most. Go farther and your cutin rises enough that you don't charge - or don't charge much - even in typical winds, again reducing the power collected.
So there's an optimum happy medium range that you try to hit, to match the genny/battery-voltage combo to the turbine and the typical distribution of wind speeds it encounters.
