Idioms - 2

Continuing the Idioms Post ...
Case :1.
I came across this Question with the 2 closest possible choices I had narrowed down to :
The gyrfalcon, an Arctic bird of prey, has survived a close brush with extinction; its numbers are now five times greater than when the use of DDT was sharply restricted in the early 1970’s.
A. extinction; its numbers are now five times greater than
B. extinction; its numbers are now five times more than

Now there has to be some pre-defined rule in English for this because without it, the Colloquial English has made both the answer choices equally probable specially to the non-native speakers like us.

Now, the rule book says - More than is for Quantity & Greater than is for Quality.As explained by the instructor herself here. In a set of 5 nos {1,2,3,4,5} if I add 6th no as 6 {1,2,3,4,5,6} I can say 2nd set has greater numbers than 1st.So greater is for - Number of Numbers, i.e to show/compare their Quantity.

So, in this case, as we clearly have to talk about the numbers of Birds, it has to be GREATER THAN.
You can find more discussions (official ones) about it here.