I've been both. At my heaviest, I was wearing a 40DD or so. Nowadays, 32A. I much prefer being smaller. A lot of tops are made for a bigger chest than I have, but it's not a big deal. There are plenty of bras out there that'll make you look a couple cup sizes larger, when it's needed...

And if being healthy and fit means being flat-chested for me, so be it. It's a very small price to pay.