Gary, sunlight is the whole key to bulb size---presuming that the garlic was grown with the proper nutrients.

A bulb is merely ("merely, he says. Ha!) a plant's way of storing sunlight. The amount it gathers is, of course, a function of the amount of green surface exposed to the sun, and for how long.

Thus, you provide the right growing conditions so that the plant grows as many leaves as it can, as large as it can. Then, during the bulbing period, those leaves gather the sunlight and store it in the cellar for later use.