My wife's overall build is very similar to Laura San Giacomo's and I've seen my wife's size vary as dramatically as is seen in the pictures in this thread. And the bra itself can make significant differences.
My wife was at DD when we were first married, and with pregnancy she went up and down quite a bit, topping out at an H and then coming back down to DD to E. Her size can change by at least a full letter up or down depending on what's going on, cycle, activity level, angular moment of spinning tops in Kazakhistan, and whatever other mystical breast-resizifying powers exist...
This doesn't even reflect changes she's experienced due to wait loss or gain over the years, which have been even more dramatic. In my experience with her and past girlfriends, it seems to me that larger busted women change size more apparently for various reasons than smaller busted women.
But when it gets down to it all I can say is, "Hooray for boobies!"