Regarding - "they are fairly full, but due to the heaviness, I have ptosis".
The appearance of the breasts is at its most basic a inverse relationship between Breast Fill and weight and the Breast skin envelope; If the skin envelope is unchanged and the fill increases, the breast become fuller (but with time, skin gives and these larger breasts droop, as is seen with virtually all women). If breast fill decreases and skin stays the same (IE not breast reduction or Breast lift hems it and takes excess skin off), the breasts droop (rock in a sock look in advanced cases). This is very similar to a sail boat on the waters. When the wind blows, the sails fill up, but when the wind goes away (no fill in the sail), the only way to get the sails flat is to roll them up.
In VERY MILD droopiness, a moderate implant MAY lift the breasts up SOMEWHAT. But, this lift is temporary and as the skin gives and relaxes, the breasts WILL progressively sag.
When the breasts "they are fairly full, but due to the heaviness, I have ptosis", ADDING to the already heavy breast with an implant will NOT work. It will merely stretch out the skin, worsen the droopiness AND compromise the blood supply to the skin envelope so when you change your mind and decide to have either a Lift OR Reduction, you may have a higher rate of potential nipple death and wound separation.
Putting in an implant in your case will not benefit you and may hurt you in my opinion.
I hope this was helpful.