Why are the noble gases so small using relative atomic size and so big with van der waals atomic radii

I just got a question on my gcse revision paper about the trend in relative atomic size across period 3, from sodium to argon, and i don't recall ever being taught ANYTHING about the relative size of atoms, but i know that using the metallic and covalent radii measurement method (distance between two bonded atoms divided by 2), and obviously, noble gases can't form bonds, so they used the van der waals radii method (never heard of it) and what they came up with was that they follow the trend the entire way across until argon, at which point it greatly increased, yet on relative method, it got smaller and followed the trend... OH SCIENCE GODS... BATHE ME IN YOUR KNOWLEDGE!

or just answer me... whatever is less trouble =D

Well the relative mass increases so the molecules get bigger blah blah blah increased van der waals forces blah blah blah shielding effects blah blah blah. Argon is bigger than expected cos it has a full outer shell or some **** like that.