Breast measurements will help you to determine whether the size you've chosen will fit well within your chest.
Some breasts are set low on the chest wall & require taller implants to create upper chest fullness. Others are deflated after pregnancy or weight loss and simply need to be filled to their previous volume. Other still may have degrees of sagging too severe for implants.
Your breasts are not identical. One may be larger, wider or set higher on the chest. This is normal. The difference may be minimal or severe. Implants may reduce, but not eliminate the difference. Breasts stretched to different volumes with implants will not feel exactly the same.
The space between the breasts may be narrow or wide. The skin and subcutaneous fat are very thin in this region. Cleavage is generated by pressing the augmented breasts together with a bra or bathing suit, not by forcing the implants together at surgery.