Hell no. Cup means only cone's height. If you change the base, volume changes too.
34C is smaller than 38C.
44A is bigger than 30J in volume, but is wide and flat.
90% weight of the boobs should be supported by band. Straps are only to make it all stable.
So you shouldn't wear bigger band size than you're under bust. Choose smaller. If you're 76 cm, choose size 70 (32), not 80 (36).
See this: http://www.atlastravel.nazwa.pl/mini/
Under: very tight
In: loose
Convert to cms and check the size.
I explained it all a lot here: http://www.wrongplanet.net/postp2773662.html#2773662