Volume vs. weight and breast implants
First of all, all the implants are marketed by volume, not weight. For saline, one cc of volume weighs one gram. For silicone gel, that is almost correct, though the one cc of gel weighs weighs slightly less than one gram (0.98 grams to be exact)
A 400 cc gel implant has 400 cc of silicone gel. A 400 cc saline implant if filled to that amount also has 400 cc. The entire implant will, however, weigh a bit more, because the shell has some weight also, averaging about 20 grams...