I'm sure it is possible to put more than 300 ccs but I'd have to examine you. It's based on the width of your chest, the look you want, and quality of your skin.
For long term natural looking breast and to protect native breast tissue, implant size is limited by your tissue characteristic like breast width, native breast tissue, skin tightness.
if 4 plastic surgeon says for limit you have to accept it, as personal examination is the final judgement.
You can go for larger implant after 6 to 9 months afte primary surgery.
You are limited in the size you can go by
1. The width of your chest.
2. The amount of skin available.
You simply cannot fit an implant much larger than 300cc in such a tight space. The bigger you go the more risks there are of capsular contracture, drooping, rippling and problems with wound healing.
think carefully before proceeding and see a board certified surgeon
Breast implants: You may need a second surgery to get all the size you want
When I have a patient who wants more breast volume than I can fit into their frame at the first operation, I advise them they might need a second surgery after nine months. At that time the tissues have been stretched and may allow me to place a larger implant. There is a limit as to what size can fit. The doctor needs to be able to close the incision. Hope this answer helps you and good luck. For more information on this and similar topics, I recommend a plastic surgery Q&A book like "The Scoop On Breasts: A Plastic Surgeon Busts the Myths."