I've always assumed 'cleavage' was a reference to breast size (making an ass of me if not u). But when researching what the gap between the breasts is called, I discovered that's actually the proper meaning of 'cleavage'. Logical if you think about it, like I didn't!
Yet when I searched for images of "impressive cleavage", most of the images showed big boobs that were touching or millimetres apart. So I have to conclude that much of the world is as dumb as me and if I use the term 'cleavage' in a story, at least some readers will assume 'big boobs'.
On the other hand, no SOL stories contain the words "intersternal gap".
Authoring sure ain't easy ;-(
AJ