I have had the operation twice now, first time was just lypo and the second time was lypo and gland removal (90%).
The doctor the second time is a well respected specialist in this area (and exceptionally sexy......yes she is female) and more or less told me that there is no way that she would ever do a full removal as the chances of the nipple 'dying' would be quite high.
She removed as much as she dared knowing that I would probably go back to using gear however stuck by her word and still left what she thought was the minimum. She also told me that the glands would grow even without gear, training, beer and poor diet would all contribute.
I am suspecting in your case it is just natural growth (understanding that you have not returned back to using) caused by training alone