Hun, you might not feel like it because you got a 'small' back size. If you would have been a 40D or something, it would look way different. But your size really is above average, although you not even feel like it.
On top of that, 36D and 34DD might be the same cup size, but it's not the same size! You need to make sure, you can't pull the band away too much, not more than 1-2 inch. Try your bra on the loosest hook, so you can tighten it up.
Of course the cups shouldn't be wrinkly and you shouldn't have any extra fabric left. If the wrinkling however is only on the top of your cups, it's because the bra is too small and the cups don't fit around your breasts.
If your band is too loose, it might feel 'more comfortable' but your breasts will sag and you won't get any support. 80% of the support comes from your band, so it isn't supposed to be very loose anyway! Also, make sure the straps don't dig in, not even a slightest bit. If they do, your band size is too big.
Good luck with finding YOUR size.