Here's a UK size chart which gives an idea for understanding cup sizes. It has to be noted once again that the cup letters are not something universal, they're nothing but a ratio of cup volume to band lenght. Also breast shape is a big factor on how breasts look like and how bras fit, what fits by measuring tape might not be what actually fits the best and very often cup is shaped wrong, its too deep or too shallow, wire width doesn't sit in right place. Bra should be like good shoes, made fot the wearer.
Here's a quick conversion table to understand the differences of most common sizing systems.
Idea about breast shapes, note that width and shape of the chest behind makes a lot of difference, this one shows what different root widths do to the shape.
This doesn't go through chest shapes, a lot of trans women and men have wide chests, so east-west shapes are common, with wide root that makes breasts look much smaller than they actually are. That's a thing with wide roots almost always except when they're also very projected which is a rare combination. Then there's different fullness in shape which is yet another factor on on proportions.
Most guys will never be clocked as having anything else than moobs or pecs, even if they're quite big. Especially on a chubby dude, they will never register unless they're huge which is likely to never happen. But a slimmer guy with narrow chest and very round and full shape will have harder time trying to fly under the radar.
When I was still hiding my transition and wearing 80F (EU) bra, no one had faintest idea of anything. That's quite a bit of boob already, but taking in consideration my height, size and breast shape, its no wonder. I think the point of no return is when they're as wide as your face, stick out from below any clothing and have obviously round full shape. And wider than chest from front is something no man ever has. Cleavage is of course super mega obvious tell.