here is a link that explains proximity effect very well. as you can tell i'm too lazy to explain it all myself
but links are always useful for the site and its members in general
but here is the paragraph that sums it up much better than i could have typed it off the top of my head.
"When you get close to a cardioid or bidirectional mic, the bass is exaggerated. Why does this happen? First we have to examine what is making the microphone diaphragm move in the first place.
Pressure Gradients <<<<<<<<<<<<<<< BINGO!
The diaphragm will move when the pressure behind it is different from the pressure in front.
If we have an omni mic, the pressure behind the diaphragm is always the same because it is enclosed. On a cardioid or bidirectional mic, the pressure on the back changes just as the front pressure does, but a bit later, because the sound waves have further to go, traveling around the diaphragm, and through whatever delay structure is there to provide a cardioid pattern."
http://arts.ucsc.edu/EMS/Music/tech_background/TE-20/Proximity_Effect.htmlword
Brian
edit: the bingo part and bolded text