It is a thick, waxy leaf. The thick, waxy cuticle reduces water loss. The leaf is thick and the edges are smooth, which means the surface area is small and evaporation is reduced. This leaf is designed to conserve water. If you showed me that leaf, I would say that it came from a plant where water was a limiting factor in growth.
Water is vital for growth.
Plants grow in two ways, cell division and cell expansion. Cell division creates more cells and cell expansion is the increase in size of the individual cells. Cells grow by taking up water. The cell swells by taking up water and pushing against the inside of the box causing it to swell and increase in size. Young cells expand and produce new cell wall at the same time.