First off, I understand that specific ab/core work sin't really necessary if you're doing compound lifts, which I am, and that diet/leanness is the real key to great abs. However, I happen to really enjoy ab work for its own sake.
That said, I'm a very lean female (about 18% BF) and while I want to have a sculped 6-pack, I don't want to make myself bulky around the waist. I've read that core exercises involving movement (Roman chairs, sit-ups, hanging leg raises) will enlarge abdominal muscles rather than merely toning/increasing muscle density. By contrast, isometric core exercises (planks, renegade rows, L-sits) will strengthen without bulking.
Is this true? Further information? (I really want to do HLRs, dang it, but not if it'll turn me from a wasp-wasited vixen into a brick!)