The band is pretty much the same in both bras. The 30DD bra has deeper cups than the 30D bra, but they are about the same width.
Comparing a 30DD with 30D in Gossard Glossies Moulded Bra (6271
30DD - Cleo » Juna Balconnet Bra (6461)
Non Padded Glossies, Moulded Bra - White
Gossard Glossies Sheer Molded Bra 6271
The Gossard Glossies Leopard Sheer Bra 13101 is a shimmering, molded leopard print underwire bra with plunge front. Gossard's Glossies Leopard Sheer
Glossies Leopard Sheer Bra
Gossard Glossies Sheer Bra
Gossard Womens Glossies Lace Sheer Moulded Bra, 30B, Nude
30DD - Cleo » Marcie Balconnet Bra (6831)
Non Padded Glossies, Moulded Bra - Black