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Allometric relations between two observables are a widespread phenomenon in biology. The volume of nuclei, for example, has frequently been reported to scale linearly with cell volume, V_{N}∼V_{C}, but conflicting, sublinear power-law correlations have also been found. Given that nuclei are vital organelles that harbor and maintain the DNA of cells, an understanding of allometric nuclear volumes that ultimately define the concentration and accessibility of chromatin is of great interest. Using the model organism Caenorhabditis elegans, we show here that the allometry of nuclei is a dynamically adapting phenomenon; i.e., we find V_{N}∼V_{C}^{α} with a time-dependent scaling exponent α (“dynamic allometry”). This finding is due to relaxation growth of nuclear volumes at a rate that scales with cell size. If cell division stops the relaxation of nuclei in a premature stage, α<1 is observed, whereas completion of relaxation yields α=1 (“isometry”). Our experimental data are well captured by a simple and supposedly generic model in which nuclear size is determined by the available membrane area that can be integrated into the nuclear envelope to relax the expansion pressure from decondensed chromatin. Extrapolation of our results to growing and proliferating cells suggests that isometric scaling of cell and nuclear volumes is the generic case.