How much fat is typically retained after a penile fat transfer and can overfilling be done to make up for absorption loss?

It's very hard to estimate how much fat is retained because there are multiple factors that play a role in how much fat is retained post fat transfer. The age, health of the person that is having the procedure done, whether they smoke or not matters. Type of suction machine used to extract the fat, the technique of the doctor doing the procedure. The time between suctioning the fat and putting the fat in the penis matters. Because of all the above factors, and then some, enumerated above it is hard to estimate the percentage of fat loss. In our clinic we use a laser liposuction device called Beautifill that removes 95% of the fat intact and so our aim is to get fat retention of 60 to 80 %. Overfilling is counterproductive because the blood supply to the area will not be sufficient if too much fat is packed in, leading to loss of all the fat.

In general, when doing fat grafting/transfer most surgeons try and overdraft to account for the percent “take,” which is quite variable. However, given the location and recipient site limitations, the amount of “extra” fat that can be grafted has significant limitations. Graft too much and you can have complications such as skin or fat necrosis.
To answer your second question, once incorporated, the fat acts like any other fat in your body. If you gain weight, the penile fat can increase, if you lose fat, the penile fat volume can also decrease.As always, I recommend seeking consultation from a board certified plastic surgeon. Best of luck on your journey!