Answer :

It happens due to the inner resistance. Resistance causes heating of tungsten filament and with more resistance, a light bulb produces more light.

Explanation:

As the current in a light bulb increases gradually, the filament shines more and more brightly. This occurs as the more current passes through a light bulb, the brighter it glows.  

The more the current, the more will be the power, here the power determines the brightness of a bulb. More the current, the more will be the dissipation of power in the form of light and heat. If the current enhances then the brightness will also get an increase.