Body size is a key life-history trait that influences community assembly by affecting how ecological processes operate at the organism level. However, the extent to which the relative influences of ecological processes mediate the assembly of differentially sized soil organisms is still unknown. Here, we investigate the community assembly of differentially sized soil microorganisms and microfauna using a continental-scale sampling effort combined with a global-scale meta-analysis. Our results reveal a general relationship between organism body size and the stochastic-deterministic balance operating on community assembly. The smallest microorganisms (bacteria) are relatively more influenced by dispersal-based stochastic processes, while larger ones (fungi, protists and nematodes) are more structured by selection-based deterministic processes. This study elucidates a significant and consistent relationship between an organism life-history trait and how distinct ecological processes operate in mediating their respective community assemblages, thus providing a better understanding of the mechanisms supporting soil biodiversity.