Answer:
light
Step-by-step explanation:
The factor of photosynthesis that is maximized by the production of large, thin leaves that span over large areas is light.
Light in the forest floor is limited due to the shade provided by the overstorey layers of the forest. As such, light becomes a limiting factor for photosynthesis for forest floor dwelling plants. In order to overcome this challenge, forest plants increase their leaf surface areas and thickness so as to increase light-gathering efficiency.