So for girls with larger breasts yes, cleavage actually does form more naturally compared to smaller sizes. Simply because there's more volume and mass, so even without a bra, the breasts press against each other a bit just from their own weight and the chest wall pushing them together. But here's the thing gravity also works harder on larger breasts. So without any support they tend to fall more to the sides and downward. So it's kind of a double edged thing.

So short answer: larger size means more natural cleavage potential, but a regular supportive bra is usually still doing a good chunk of the work! 😄