lipid bilayers are impermeable to ions so ions must pass through ion channels. smaller ions attract water more strongly because its charge is more highly localized. this effectively makes a larger water shell and means that smaller ions, like Na+, will behave like larger molecules compared to larger ions, like K+.
size of the molecule still doesnt tell the full story. see pore theory explains channel selectivity
references
Kandel, E. R., Koester, J., Mack, S., & Siegelbaum, S. (Eds.). (2021). Principles of neural science (Sixth edition). McGraw Hill.