I think the inheritance thing varies widely. I've seen several busty women whose mothers are not very big at all and I've seen mother/daughter combos with similar chest sizes.
Well, it's not so much that it varies. It's always heavily genetic, with some environmental factors thrown in. There are multiple genes that factor into breast size though. Setting aside the environmental factors for a moment, there are genetic factors that could cause a disparity in breast size between mother and daughter.
1) The mother is handing down a lot more genetics than her actual bra size. In this way a flat chested mother can give her daughter some genes that can encourage a larger chest. In theory, if a woman has a DD-cup and the daughter only gets 50% of those genes she might wind up an A-cup. With that said, she can still hand down those busty genes to her daughter, and again, in theory the granddaughter can wind up a DD with the correct combination of other genes.
2) The father's breast size genes are unknown. He gets these genes from both his parents. This can also account for both big and small differences. If a woman has a AA-sized chest but her husband has genetics with a maximum for somewhere in the neighborhood of an H-cup there are a lot of possibilities there because each daughter will get her own mix. If they have 3 daughters they may each be significantly different in size. One might be a B-cup, one a D-cup and the other an F-cup. All because they each have a different genetic cocktail.