To understand the effect of laser focal spot size on the extreme ultraviolet conversion efficiency and the physical mechanism that produces the effect, we developed a two-dimensional transient expansion model of laser ablation of planar target to produce coronal plasma by means of theoretical analysis. It is found that under condition with light intensity of 7.45×10¹⁰ W/cm², full width at half maxima of 5 ns, wavelength of 1064 nm, as the laser focal spot radius increases from 60 μm to 300 μm, the corresponding extreme ultraviolet conversion efficiency increases from 1% to 5.5%, while the corresponding extreme ultraviolet conversion efficiency stays at 5.5% after the focal spot radius is larger than 300 μm. This is due to the fact that the plasma in the coronal region generated by laser ablation of planar targets expands from the initial one-dimensional expansion to the subsequent two-dimensional expansion, which determines the saturation size of the plasma region emitting extreme ultraviolet light and ultimately determines the conversion efficiency of the extreme ultraviolet light. Our theoretical analysis on trend of conversion efficiency with focal spot radius can explain the physical phenomena observed in the laser ablation of a tin target experiment.