The radiative lifetime dependence on the dot size in multilayer ͑In,Ga͒As quantum dot structures with different thickness GaAs barriers was studied via photoluminescence. In the structure with thick barriers and isolated dots, the radiative lifetime increased monotonically with the dot size, which was attributed to the enhanced exciton oscillator strength in smaller dots. By contrast, in the structure with thin barriers and electronically coupled vertically adjacent dots, the radiative lifetime increased and later decreased with increasing dot size. This can be explained by the enhancement of the exciton oscillator strength in larger dots, which are coherently coupled through vertical tunneling.