Im feel my 300 cc implants are too small
The different implant companies are different. Some max fill volume are 25 -50 cc's above the lower number. It is not recommended by the company to exceed those amounts.
The max fill volume depends on the profile of implant. It may either be 25 or 60cc depending on the profile implant you have. You will have to undergo surgery again to increase the volume. You may not notice a significant size difference even with 60cc added and may want to increase the size of your implants. Talk with a board certified plastic surgeon.