Strain induced by external pressure in pure nanocrystalline SiC powders with different sizes beginning from 2 nm was examined with application of in situ high pressure diffraction technique in Diamond Anvil Cell.. Two methods were used for elaboration of the diffraction data: (i) strain was evaluated from asymmetry of Bragg reflections fitted with use of split-Pearson functions and (ii) strain was modelled with assumption that under pressure the actual lattice parameter of a grain depends on its size where smaller grains have more compressed lattices. The diffraction profiles were simulated ab initio from Debye functions for different grain size distribution functions and compared to the experimental patterns. It was found that transfer of the external pressure to the cores of individual grains and distribution of strain in the compressed powders depend on the size of the grains. The pressure gradient accross grain boundaries increases with the increase of the size of the grains. A model of microstructure of a compressed polycrystal presented earlier for nanocrystals with grains larger than 8 nm is here verified for polycrystals with the grains of smaller sizes, down to 2 nm