Madsometimes oh yes indeed, there can be a lot of difference between brands. Having said that, I do seem to be a consistent 30GG in the brands/styles I wear most. I'd possibly go up a cup in some Bravissimo styles (hoping to try some new ones on next trip) and I definitely need a bigger cup in many plunge styles.
Fantasie seem quite big in the band and cup, Curvy late run big in the band and small in the cup. Unfortunately you have to try every bra I usually start with my standard size a d go from there unless I've been pre-advised to size up/down.
You may not have loads to scoop but its still perhaps doing. Any underarm bulge for instance, needs to be dragged into the cup.