All depends on shoulder shape, ime. I have sloping shoulders, so no matter bra size or style, the straps can be slippy. Friends w/similar shoulder shape have the same issue (usually, it's costumers who've realized this bec. we have pattern fit problems in that area :).

A combo of larger breasts & straighter/higher shoulders (along w/well-fitted bra) will prob. lead to less slippage.