To start out, I will cite some data. Pope Paul VI decreed that the number of electors should be 120. After that, in three consistories, Paul VI created 55 cardinals (18.34 cardinals per consistory). After that, John Paul II waived the limit as he created 231 cardinals in nine consistories (25.67 cardinals per consistory). This all can be found here. I haven't found any data on what the make-up of the College was after each of Paul's consistories, but I assume he followed his own rule.
The primary purpose of the office of Cardinal is to elect the successor of St. Peter. It has gone through numerous changed throughout the centuries. Paul VI's limits are not dogma and they do not consititute a magical number that cannot be changed.
As the Church grows in numbers and diversity (ethically, geographically, linguistically, etc.), the number and distribution of this office needs to change. The Church is an organic institution. We need to be flexible so that the representation of Christ's body on earth is given a voice.
Of course it's not dogma and there is no such thing as a 'magic number'. However, observing how representative bodies interact across the world, a number that represents the best compromise between representation and efficiency can be decided upon. As it's noted, the college's size has gone through numerous changes, but as circumstances have changed, so to has the number. Just the other day, Benedict announced his hope that the college would serve as a senate and meet regularly to discuss the issues of the Church. So there are two choices. One, the college can be really large and representative, like the US House or the UK Commons with their restrictive debate rules and practices or the college can be small like the US Senate with its rules of free-flowing debate.
Given these two models, what parallels can we find in the Church itself? Just last year, we had two examples of assemblies gathering. The first was of course the college before and during the conclave. The second was the Synod of Bishops. As I noted in the other posts, there were reports of cardinals before the conclave not knowing each other and various quick meetings to figure out who was who, etc. At the Synod, there were new rules put in place to bring about more debate, but as the new rules dictated, bishops were given less time to speak, not more.
We have the College of Cardinals that be default has a limit of 120 members. These members serve collectively as the executive consultive body for the Supreme Pontiff. Then there is the Synod of Bishops, which has hundreds of members and lots of rules to make sure that everyone gets his five minutes of speech-time.
Your argument that around 100 cardinals is good because it allows them all to know each other, from a non-theological and practical point of view, sound deceptively like a great idea; however, it lacks a theological motive or justification.
I don't think any modern pope has created Cardinals just for the sake of makign Cardinals. I think John Paul II created a marvelous precedent. He greatly diversified makeup of the college so that the needs and concerns of some of the poorest local churches could be voiced at the highest levels.
The offices of the Church, in the end, serve a pastoral purpose. The needs of the Church, for the sake of the salvation of souls, is the highest law of the Church. It is in this light that we need to reflect on the number of Cardinals we need in the Church. The needs in the late 60s are not the same needs as the church in 2006. A lot has happened, my friend.
First of all, that kind of theological motive is needed? As the reader noted, "The primary purpose of the office of Cardinal is to elect the successor of St. Peter." When all the cardinals were pretty much Italian and more generally European, a large number was tenable since due to geographical proximity, everyone knew or had at least heard of each other. However, now with cardinals scattered to the four winds, this rubbing shoulders is not as easily done.
One can either have a huge college that is basically an example of senior cardinals stage-managing the lesser cardinals into massive voting blocs based on similar thinking and geography or one can have a smaller college where a hundred or so cardinals know each other intimately and know what kind of man each cardinal is deep down.