Mariana,
If you are unhappy with the fuller implant you should easily be able to drop down a size to the 425cc implant that you suggested. This may be just enough to please you yet be only a modest reduction in size.
The firm feeling may also be a consequence of the high profile implant. I know they are very popular where you live, yet the high profile contour is almost a round appearance which lack flow with the breast tissue, and give a look to the breast rather like, well, a breast implant. Natrual is not the first word I would use to describe them and a lower profile implant, perhaps a moderate profile plus may have a softer feel in a 425cc gel that the higher profile style. The projection of the implant may be what appears "big" to you so be sure to discuss this with your surgeon.
Take your time deciding as there will be little risk changing the imlants.
Best of luck.
