Rather mysterious mitigators

I found this important material about mitigators. I think this material is really (rather? asks a student like me struggling to choose between ‘rather’ and ‘really’) important for those who try to cope with managing adjective intensity.
But, still there is a puzzle:

lessen the noun`s intensity

lessen the noun`s intensity

“Mitigators are the opposite of intensifiers. When we want to make an adjective less strong we use these words:

fairly – rather – quite


We use these words and phrases as mitigators:

a bit – just a bit – a little – a little bit – just a little bit – rather
This one is rather bigger (Mitigators with comparatives).”

Now there is no logics here. If something is BIGGER, then there is more of it, but the mitigator RATHER lessens the quality of quantity superiority.
Thus, saying ‘This is bigger’ me strengthen the superiority, but saying ‘This is RATHER bigger’ we lessen the superiority while making it NOT superiority, but inferiority!
I doubt if anyone would say ‘this is rather bigger’.
This looks like nonsense, and if you`re a British native, I need your advice, as grammar is so descriptive, and must need some explanation from the side of a thoughtful native.