the latter has no definition. but i could infer that breast expansion somehow involves showing/implying growth of breast from small to larger size like post #61100; and featuring characters with canon small bust size like post #690605. however, the second qualifier is somehow difficult to those unfamiliar with the character. which are we going to adapt?
i personally think bursting breasts and breast expansion are distinct, as the way posts are handled. but it requires some cleaning hence to set the term for breast expansion.
btw, is it applicable to imply breast expansion -> bursting breasts?