We use these words and phrases as mitigators:
a bit
just a bit
a little
a little bit
rather · slightly
just a little bit
She's
a bit younger than I am.
It takes two hours on the train but it is
a little bit longer by road.
This one is
rather bigger.
We use slightly and rather as mitigators with comparative adjectives in front of a noun:
This is
a slightly more expensive model than that.
This is
a rather bigger one than that.