The brightness of a light bulb is primarily determined by the amount of current flowing through it. In parallel circuits, the total current can be greater than in series circuits, which explains why additional bulbs do not cause dimming. Although current is shared among branches in parallel circuits, each bulb can still receive sufficient current to maintain brightness. The wiring in these circuits is designed to minimize voltage drop, ensuring consistent voltage delivery. Understanding these principles clarifies the relationship between current and brightness in different circuit configurations.