Subject: Van der Waals constant (b) for neon

Dear Mad Scientists:
     Most topics in the AP Chemistry curriculum are fairly unambiguous 
and easily answered using basic chemical principles.  While teaching 
the Van der Waal's equation for real gases, I came across something i 
did not completely understand: If the "b" constant is correlated to 
molecular volume, why would the b value for neon be smaller than the 
value for H or He?  I checked the Handbook of Chemistry and Physics, 
and the closest I could come to an explanation was the fact that the b 
value was also correlated to a compressibility factor.  
Could you please help?  My students and I would really appreciate an
answer.

Re: Van der Waals constant (b) for neon