It is a myth that breast implants alone will create significant cleavage. Cleavage depends on:
1) The shape of your breasts
2) The location of the breasts on your chest
3) The diameter of your breasts preoperatively
4) The degree of breast augmentation
5) The amount of breast tissue that you have
6) The size of the implant
It is a myth that breast implants alone will create significant cleavage. The profile of the implant can be helpful. Implant profile should be based on your chest width. For instance if you have a narrow chest and place a low profile implant you may have better cleavage but the breast implant may also be located to the outside of your chest. Conversely if you have a wide chest and have placed a high profile implant you may have minimal improvement of your cleavage
If you have an average chest wall size, there is a better chance for having improved cleavage. Especially if you do postoperative muscle stretching exercises. However, it is not a guarantee and just increasing the size does not necessarily increase breast cleavage. It is actually can be the opposite: since the breast implant can actually push the breast to the side resulting in decreasing the amount of cleavage.