The large plant has more mass that must be maintained and needs more oxygen to do so. But it has less surface area per gram of body weight that it can use to exchange gases. But on the positive side, it also exchanges less heat with the environment, because heat also is exchanged across the surface. Thus the large plant will heat up and cool off more slowly than the small plant. Of course, shape also matters.
Different shapes have different surface area to volume ratios.
The rounder an organism is, the less surface area it has for a given volume. For example, the next graph compares the surface area and volume of a cube and a sphere. At any given volume, the sphere has a lower surface area than the cube. This difference increases as the volume goes up. So, an organisms can change the ratio simply by changing its shape.