How cup volume scales
Each band size adds about 0.5 inch to cup width. So 32D and 34D look similar on paper but the 34D has a wider, deeper cup. This is why sister sizes work — going down a band and up a cup keeps volume constant.
Why this matters when shopping
If a 34C does not fit, try 32D (sister size, more projected) or 36B (sister size, wider and shallower). Same volume, different shape.