Its completely normal to not be leaking.. Not everyone leaks while still pregnant, some don't start until after giving birth, could even take a couple days after giving birth. And you're still just 20 weeks, your breasts still have plenty of time to grow and even if they don't, its not a horrible thing. Try not to worry so much :)
Not everyone leaks and your breasts still have plenty of time to grow.
I'm 39 weeks and just started leaking but my breast size is the same everyone is different