It is interesting to note in this regard that
http://en.m.wikipedia.org/wiki/Szlachta
That is comparable, in England at the same time, not with the size of the nobility, which was very much smaller, but the number of Englishmen eligible to vote, as a proportion of all men. (In Scotland the electorate was much more restricted, even after the Union of 1707).
So, the size of the citizen population in Athens was comparable with the size of the largest enfranchised social groups in 18th century Europe.
Is this the maximum it can in practice attain in pre-industrial society?