Various properties of multi-principal element alloys (MPEAs) are attributed to local lattice distortion caused by mixing atoms with different sizes. Whereas the idea of atomic size has been very widely and successfully used in many fields, it is not so obvious how it should be defined, given that the electron density is continuous. It is usually defined empirically to explain the bond length, assuming transferability from a system to another. However, there is a serious problem with transferability, because chemistry can affect the effective size. It is well-known in chemistry that the ionic size depends on valence. In the same way charge transfer affects the atomic size in MPEAs. We discuss the results of the DFT calculation on the atomic-level pressure to determine the effective atomic size in various 3d-transition metal MPEAs, focusing on the effect of charge transfer. The work was supported by the NSF, DMREF-1921987.