To your question, I will write a slightly longer answer.
At first I overestimated myself. Because it’s not all in the size, but it’s also in the shape of the bra, the depth and width of the cup, etc. As I was buying bras online, I couldn’t know what kind I was going to get. I measured myself, but obviously that’s not the only criterion.
I bought my first bra in size 40B. He was white. When I put it on, it was quite big for me. There were gaps and that bothered me.
Then I ordered 34A. He was black. The tape was shorter, but it didn't bother me that much, because the cups held my breasts well. When it got tight, I bought a third bra. Dark blue. He was size 38 B. That bra was the most comfortable for me. The strap was soft and stretchy and I didn’t feel it at all (same as with the 40B) and the cups held my chest well. Over time, I felt my breasts begin to protrude beyond the cups, meaning I had outgrown it.
And recently I went back to the first bra, white, size 40 B.
There are no more gaps and my breasts fill it almost completely. The left basket is all filled, and the right is not. But there is very little space. That white is the most comfortable for me so far and I don't feel like wearing it at all.
I took pictures with all three bras, so you can see the difference.