Ok, cup sizing... the bane of all things sane.
By the usual measuring advice, I should be an F cup when swollen and D as base levels. I am obviously not. So the first trick is to find how much "off" you are from the usual guide.
Now the "1 inch should be visible?", you need to understand this 1 inch is a relative change for when a new cup size happens. For visibility, we have to put that 1 inch in terms of geometry. I'm going to make this very simple to illistrate but I think you'll get the idea.
I'm going to make your torso a circle rather than the odd ellipse most humans prosess. Thus, your 28" underbust is now the circumference of a circle.
2 * pi * r = 28, thus r is about 4.46
Now, let's add that 1 inch (an A cup)
2 * pi * r = 29, thus r is about 4.62
That's a different of 0.16" in the radius. And to help give meaning to that measurement.
0.16 = 4/25 < 5/25 (aka 1/5")
Thus, can you see a change less than 1/5" on your sides? With an ellipse, the numbers would be smaller still. Even if I use 35" and 36", I still get 0.16 difference in the radius (closer to your bust measurement).
The reason we sometimes see a big change is due to volume increases rather than bust vs underbust changes. There is why I mentioned the "off factor" for cup sizing since we don't use volume in our standards.
I hope that helps understand the visibility (volume increases way faster than circumference) of changes.
DJ