The secret is all about how tight you can get the binding quickly and the problem with rope is that it's not ideal for a fast binding. As you've discovered once you get a globe effect the binding is held in place by the groove behind the bulging boob.
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As seen in this example, even the smallest tit can be bound if you are determined.
And this is how you can then enjoy rope bondage, by using something quick like the cat collars, or as suggested by
@subzzzero electrical tape or zip ties. With your boobs already in the right shape it's then simple to add a rope bra over this. You do of course need to keep a close eye on the total time bound and allow for removing rope before the base binding.