There's also a passage in the novel where it talks about how Tom learns the extent of Don Vito's business dealing, which, according the book, must have been huge. Much larger than Don Vito ever seems to let on.

Perhaps it were those business dealing, hidden business partnerships with high ranking political bosses, Labor union manipulation, that Puzo was alluding to?

It would seem though that Don Vito's power must have been localized in New York though given the way Senator Geary tried to shakedown Michael at the beginning of GF2.