Dear Sane,
Sorry I don't see the comparison, I am also bemused by your appeal to numbers, I could use that argument but only to show that Christianity is and always has appealed to the masses.
Christianity is a very inclusive religion, open to rich and poor, in fact one of its great appeals is that your status in life counts for nothing.
So even if this Constantine bloke did not have a vision it would still have become one of the greatest religions this world has ever seen, so luck or Divine intervention.
Gonnagle.