Isn't it more like:

  N-illion = (Size_of_Digit_Grouping)^(n-1)

Since the OP was saying groupings use to be by 6-digits, not 3-digits (i.e. "1,000") as seen today.

The original long scale makes more sense, billion, bi million, twice six digits.

Yet, mille and milli- mean thousand.

But the suffix -one in Italian is used to indicate something big - spaghetti vs spaghettoni - so a milione is a big thousand, a million.