We study the deviation from diffusion theory that occurs in the dynamic transport of light through thin scattering slabs. Solving numerically the time-dependent radiative transfer equation, we obtain the decay time and the effective diffusion coefficient Deff. We observe a nondiffusive behavior for systems whose thickness L is smaller than 8l(tr), where l(tr) is the transport mean free path. We introduce a simple model that yields the position of the transition between the diffusive and the nondiffusive regimes. The size dependence of Deff in the nondiffusive region is strongly affected by internal reflections. We show that the reduction of approximately 50% of Deff that was observed experimentally [Phys. Rev. Lett. 79, 4369 (1997)] can be reproduced by the radiative transfer approach. We demonstrate that the radiative transfer equation is an appropriate tool for studying dynamic light transport in thin scattering systems when coherent effects play no significant role.