I saw a lot of people claiming that they know they shouldn't go up in cup size because when they do the bra fits worse than their current ones. This (incredibly basic) animation might help explain how this works, and why gaping is usually NOT a sign of cups being too big:
The animation starts with three different cup sizes (large, average/medium and small), and how they properly fit on breasts of the right size. Then, breast size changes to show the effect of wearing that cup size on the wrong size of breasts.
The first image shows the effect of wearing a cup that's too large, and as you can see the empty space doesn't really happen at the top but in the cup itself.
Gaping at the top (second image) happens instead in cups that are too small, as the bra is pushed away from the body by the curve of the lower breast tissue.
However in cups that are WAY too small (third image), sometimes less gaping can be seen. This is because it's physically impossible for cups this shallow to conform to the curve at the bottom of the breast, and instead it is pulled up to lay against the front of the breasts. The front of the breasts is more easily flattened to fit the curve of the cups. Additionally, because way too small cups will be smaller, the leftover difference in curvature won't cause visible gaps.
