When filled with exactly 300cc of saline, this implant will weigh 315grams. But it will be "under-filled" and will have visible rippling and possible early deflation. "Proper" fill for a 300cc implant is about 315cc-340cc, depending on manufacturer. When I use saline implants, I make the final overfill judgment in the operating room where I can assess the patient's overlying tissue (muscle, breast, and skin) tension and resultant pressure on the implants.
BTW, if you weigh yourself right after surgery you will weigh as much as 5 or 6 pounds heavier than when you went into the operating room! This is because your body retains fluid for a few days to weeks after surgery or trauma. You didn't suddenly gain weight!
Web reference: http://www.mpsmn.com/html/breast-augmentation.html