"Normal-phase silica has, on average, 50 to 60 Å pores (and a surface area of 500 m²/g). However, for small-molecule reversed-phase chromatography to be useful, the average pore size must be larger (90 to 120 Å). The reason for this is that the bonded phase (typically C18) takes up space on the silica surface and can block access to the pores (and the majority of the available surface area). So, using slightly wider pore diameter silica as a support for the bonded phase improves surface accessibility for most small molecules."
Flash purification specialist Bob Bickler discusses the impact media porosity can have on chromatographic performance on the Flash Purification Blog.