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.
In a succinct reply
Night Lord has encapsulated what was discussed earlier, good job. This could be further reduced to the general statement that a daughter will tend toward her mom with a random element coming from dad. The more like her dad the more likely her bust size will be different in some way. I just liked researching and finding out why this is so.
Other threads have danced around with this topic:
- The major genes associated with breast size and growth were isolated a few years back, yes there are several and they all interact.
- Recollections of size disparity between mothers and daughters.
- I believe that some of the big vs. little busts thread are actual mother + daughter images.
I know of one friend who is of medium/small bust size and each of her three daughters are significantly different sizes, all in their late 20's or early 30's. In other words her genetics do not have much expression.