My body kinda confused about cup sizes

Okay so I’ve been fitted a thousand times at all different bra shops and I always get the same answer that I’m a 10E

but the thing is my boobs look quite small without a bra like and even when I’m wearing a bra girls I know with a D cup their boobs look so much bigger then mine

everyone I know with bigger boobs like DD , E and F cups and they all laugh and don’t believe that I’m an E and my boobs look smaller and I just don’t understand it makes 0 sense to me how can I measure up to an E but people with smaller cup sizes look so much bigger then me

Please help me understand