Usually, mugwort does not produce very large flower bones. Some of the conditions that may cause unusually large blooms include:

Cultivar: Some varieties of wormwood are inherently large when they bloom, such as sweet wormwood and bitter wormwood.

External environment: if the environment in which the wormwood is growing is well protected and cared for, they may be healthier and may produce larger than usual flower bones.

Light: Mugwort needs plenty of sunlight to grow healthy, if they get enough light, they may produce larger flower bones than usual.

About mugwort in the month of Dragon Boat Festival, normally they don't bloom because this is the month when the mugwort is harvested and dried, not the growing period of the mugwort. Therefore, if you see large flower bones blooming on mugwort in the month of Dragon Boat Festival, this may be due to other factors.