We demonstrate a statistical method to describe the fragment formation in rapidly expanding nuclear matter. The production of light nuclei in central collisions of 132Sn+124Sn and 108Sn+112Sn at 270 MeV/nucleon has been studied in the framework of the statistical model with the hypothesis of local equilibrium. The model suggests that the nuclei are formed at subnuclear density from the dynamically produced baryons, which can be subdivided into the local highly excited clusters. The nucleation takes place inside these clusters during the matter expansion, when the clusters enter the phase coexistence region of the nuclear liquid-gas phase transition. This process is described within the microcanonical version of the statistical multifragmentation model (SMM) as a decay of hot clusters into final nuclei. We emphasize the importance of studying nucleosynthesis in this energy range because of the expected change of the fragmentation mechanisms, which was also observed in previous experiments. There is a qualitative agreement with the recent experimental data obtained in these reactions at the Radioactive Isotopic-Beam Factory (RIKEN).