משנה מסכת ברכות פרק ז
ר' יוסי הגלילי אומר לפי רוב הקהל הן מברכין שנאמר(תהלים סח) במקהלות ברכו אלהים
ה' ממקור ישראל
אמר רבי עקיבא מה מצינו בבית הכנסת אחד מרובין ואחד מועטין אומר
ברכו את ה'
רבי ישמעאל אומר ברכו את ה' המבורך
...R' Yosi of Galilee says: The blessing corresponds to the size of the crowd, as it says (Tehillim 68) 'Bless ye God in full assemblies, even the Lord, ye that are from the fountain of Israel.'
R' Akiva says: It is just like the precedent that is set in our synagogues - whether there are many or few we say 'Bless ye The Lord'
R' Yishma'el says: 'Bless ye The Blessed Lord'
This Mishna (Berachot 7:3) is at the conclusion of a list of different formats for making a mezuman. There's a different format for 3,4,10,11,100,101,1000,1001,10000, and 10,001 people bentching together. The Gemara presumes that the entire list accords only with the opinion of R' Yosi, who believes that the benediction ought to reflect the size of the crowd. R' Akiva and R' Yishma'el, whom the halacha follows, conclude that once a quorum is achieved, the liturgy doesn't recognize the size of the crowd.
The way I understand this dispute, R' Yosi was, understandably, focussed on the magnitude of the event as reflected in the #s. It's a very intuitive and easy way to measure success and quantify an experience. we all do it.
R' Akiva and R' Yishma'el insist that quantity is not the way to measure a religious experience. A minyan is a body that represents all of Israel (relate the laws of Kri'at Ha-Torah to Nechemiah 9-11; Torah reading must be 'public' in order to demonstrate that the torah is public property. 10 adult males constitute that 'public' symbolically. I believe that RYBS has a piece on this in Shiurim Le-zecher Abba Mari Z"l - if someone wants to post an exact mareh makom, by all means). Once there's an acknowledgement of the 'publicity' of the event, it's as though all of Israel is represented, and greater #s don't make it more representational in that respect. #s may be valuable in other respects, but ultimately the #s alone, quantity for quantity's sake, isn't significant in the halakha and its underlying values, at least according to the way we pasken.
Phoebus connected this to the two givings of the luchot in the parshiyot that we read recently. The massive 'publicity' that surrounded the first giving resulted in a type of evangelical quantity-fetishism (my term - almost as good as new-age eco-feminist Judeo-Wiccans, no?) that ultimately led to the people building a golden calf. The second luchot, by contrast, were given privately (be-tzin'a) and stood the test of time
Compare also to the incident w/ Elijah on Mt. Carmel, where God was the greater evangelist on that day, but Ba'al ultimately captured the hearts of those who look for evangelism; shma minah - Judaism isn't an NCSY Shabbaton. Evangelism and Judaism is a bad mix. It's not what we're really about, and we can't beat the Bible-thumpers at their own game anyway (just like God's representative - Elijah - 'couldn't' defeat the Ba'al Movement (oy, vey; terrible pun; but letzonusa de-Avoda Zara is permissible. Just ask the Godol Hador) at this game.
And besides, Phoebus also pointed out (at the last Siyum 7.5 years ago) that the ma'ariv minyan at the end of the Simon & Garfunkel concert in Central Park had a much more impressive yehei shmei rabbah.