We have a ranking of countries according to the number of active members. I was interested to see how this compare with a ranking in terms of percentage to the total population. I took the top 22 countries.
I used the 2009 most recent known population census data and the values are given as permil to avoid to many zeros after the comma. Last digit is rounded. Here is what we get:

COUNTRY---------RANK MEMBER------o/oo---------RANK
UNITED STATES------------->1------------->0.2------------->11
FRANCE---------------------->2------------->0.4------------->3
GERMANY-------------------->3------------->0.28------------>7
JAPAN------------------------>4------------->0.14------------>12
UNITED KINGDOM---------->5------------->0.23------------>10
CANADA--------------------->6------------->0.31------------->5
BRAZIL---------------------->7-------------->0.04------------>15
ITALY------------------------->8-------------->0.11----------->13
AUSTRALIA------------------>9-------------->0.27------------>8
SPAIN------------------------>10------------->0.11------------>13
NETHERLANDS-------------->11------------->0.25------------>9
CHINA----------------------->12------------->0.003----------->17
CZECH REPUBLIC---------->13------------->0.36------------>4
POLAND--------------------->14------------->0.08------------>14
BELGIUM-------------------->15------------->0.27------------->8
INDIA----------------------->16------------->0.002------------>18
RUSSIAN FEDERATION---->17------------->0.02------------->16
FINLAND-------------------->18------------->0.48------------->1
TAIWAN--------------------->19-------------->0.11------------>13
DENMARK------------------->20-------------->0.41------------>2
SWEDEN-------------------->21-------------->0.25------------->9
SWITZERLAND------------->22-------------->0.29------------->6


So we see that in terms of ratio a small country like Finland tops everybody. In general smaller countries do well, like Denmark and Czech Republic, Switzerland.
Excellent score for Czech Republic if we consider data like GDP were for sure it does not compare with Finland and Switzerland or Denmark.
IMF 2008 classification ranks Czech Republic 35th in GDP/capita as Finland is 20th Denmark 16th and Switzerland 7th.

The United States score is not brilliant as it ranks 6th in GDP/capita and is the largest developed one.
In the larger developed countries France and Canada have a very coherent score as they rank difference is only one.
And finally still a lot of potential to be developed for WCG in India, Russia, China, Brazil.