How common is it for the "Bigger" (size 16+) girl to have small breasts?
Big girls small breasts
i'd say it is as common as birds in the sky. People are different in so many ways, there is no normal, boobs don't make the person, so don't worry about it.
I know quite a lot of bigger girls who have small breasts, as well as some who have large breasts. What dress size you are doesn't automatically go with what breast size you are.