Yes, but if ALL the Eastern Churches (including the non-Chalcedonians) suddenly were joined to Rome, they'd still be less than 10% of the total, due to the advanced state of decay of the membership numbers of the Slavic and Romanian Churches since the Communist catastrophe.
You are right it is pitiful. It is the result of 200 years of western inaction (or positively regressive action) led by the British in the face of the extermination of our eastern brothers by the Turks and Communists - since the time after Napoleon. How many times in the 1800's could Imperial Russia have liberated Constantinople, Pontus, Smyrna, and Rumelia if not under attack from Freemasonic Britian and Prussia and Revolutionary France? How many times did the same powers prevent Greece from her realizing her national aspirations to free New Rome and Anatolia? How many times did they ignore opportunities to smash the Communists Empire while it was still young?
So while 10-15 million Eastern Catholics may look tiny compared to 1+ billion of us Latins, keep in mind that those 10-15 million Eastern Catholics are around 15% of all Eastern Church members, due to the Eastern Churches being the front line against Islam, the Mongols, the Turks, and the Communists.
As I see it, the problem with those Eastern churches that are in communion with Rome is that they are in a limbo, between East and West. Their small number is not all that important as much as the fact that they cannot serve as a steadfast example of Orthodoxy, having demonstrated a changing heart and, in effect, admitting that they were wrong when they were Orthodox.
I hope everyone on this forum realizes that no re-union will take place unless the faith becomes one and the same and unless both side feel that they have not morphed into the other. The Church cannot admit to having been theologically wrong. Neither Church can say, "oops, we believed wrongly in this! Now we embrace the dogma of the east (or the west)."
I think you were the one who called Serbian and other such Patriarchs "petit" (a shade better than "pitiful" -- unbeknownst to most on this forum), but there is no comparison. The "petit" patriarchs are not in the same position as the eastern churches embracing papacy.
The numbers game is also interesting. Of all the people who do go to Church, as opposed to who are registered as believers, there is a tremendous gap. We know that 75% Catholics don't go to church; the attendance has dropped to ambysmal 25%, and the attendance in Orthodox churches may be even lower.
I can tell you that of the 30,000 Orthodox Japanese, claimed by the autonomous Orthodox Church in Japan, a few hundred attend weekly Divine Liturgy on a steady basis -- oh, they fill the Cathedral almost shoulder to shoulder, especially on feast days, but it is easy to see that 30,000 would not fit into that building (and there are but a few more Orthodox churches in that country -- smaller than the Cathedral in Tokyo).
Likewise, there are Catholics and there are Catholics. Those who are minimalists and those who are maximalists. I would say there are many Orthodox among Roman Catholics, and there are many nominal Orthodox who are Orthodox on paper and not in their hearts.
The numbers claimed are meaningless in terms of "true Church." It's the true Church that the Pope wants to see (re)united. The true Church cannot become absorbed, or absorb another true Church. The question is -- how do we negotiate all the various man-made obstacles that exist in order for the true Church to exist without borders (or numbers). How do we reconcile cultural, historic, linguistic, conceptual and other hurdles and, most of all, how do we overcome political difficulties in the process and resistance within each Church?
You know as I know that of the 1.1 billion Roman Catholics on the roster, the number of those who are in the "true Church" is but a small number, just as there is but a small number of true Orthodox in that same Church.
I think the Pope, however, has recognized that the unchainging character of Orthodoxy and its strict adherence to Tradition in Divine Liturgy and teaching is that rock of faith that is valuable to the Church, that resists modernism and other Church-decaying isms. The Vatican does not hide the fact that Orthodox steadfastness in guarding the Tradition is appealing.
I am surprized that, given the 500-year Ottoman rule and almost a century of godless Communism, any Orthodoxy even survived in eastern Europe. But, then we have the common history of the first three and a half centuries of Christian persecutions that only strenghtened the faith. The Church is the strongest, if not the largest, when carrying the Cross.