The same Cells add up to make a structure called tissue which it also add up to make an organ which has its own purpose or function and this organs which help each other to make a living being stay alive organs have different characteristics and the same ones add up to make an organ system, which add up to make a living being.
Plants make their food in their leaves. And need sun for this process, so smaller and fewer leaves means less sun for photosynthesis, and it also means less food to nourish the plant.