What is the real difference between 34D and 36D?
34D and 36D share the same cup letter, but the cup volume is not the same. As the band gets bigger, each lettered cup also gets bigger.
People often assume the same cup letter means the same volume across bands. It does not. 36D has slightly more cup volume than 34D because the cup widens as the band widens. This is also why sister sizes work the way they do.
How to know which size fits you
The simplest way is to measure. Take your under-bust snug and your bust loose. Round the under-bust to the nearest even inch — that is your band. Subtract the under-bust from the bust — every inch of difference equals one cup letter (1 = A, 2 = B, 3 = C and so on).
If your measurement is right between 34D and 36D, try 34D first on the loosest hook and walk around. If the band rides up, try 36D.