Believe it or not, light inhibits plant growth. Imagine a plant with a light on the left side of it. The plant will bend toward the light, right? On the main stalk, the light hits the left side, and left side is shorter than the right if the plant is bending to the left. The cells that are getting less light are actually growing bigger and faster.

Now that we're confused, plants gather the nutrients they need during lights on, and process them when the lights are off. I don't think they grow the "most" in either veg or bud, but they do of course grow differently. In veg the plants are concentrating on getting as strong and healthy as possible. During budding they are trying to get laid and focus shifts to reproduction. This means stretching to get the buds up in the air and out in the light as much as possible, and resin or pollen production.