A Fitness Model Torn Between 371 Cc and 450 Cc

I think you need some direct advice:  Reality is not all of your wish list is likely to be achievable.  If you're lean, you might well look fake.  The coverage over the implants is the key determinant of naturalness of appearance.

To assist you:  Decide how important naturalness is for you vs how important size is for you.

If you need to be more natural, go for the smaller size.  If looking implanted doesn't matter for what you do, then reading between the lines it sounds like you probably want to go bigger.

Final advice: the smaller size you're considering will look great, don't feel that it's a small implant, it's not.

Provided you have a breast width (base diameter) that will accept either of these implants, you have a decision to make.

If you are an athletic 5'6", 120 lb and you want to be a DD, then they will look fake if you go with the 450 cc. It would be hard to go to a double D and not look fake given your level of fitness. You must also take into account that you are adding this volume to an already C cup breast. If you have hips, then maybe you can easily accept the larger implant in proportion with the rest of your body.  If you have narrow hips, then maybe not.

The other thing to consider is the percent body fat you are during competition.  If your percent body fat is low during these times, the chance that your implants will have visible rippling increases. Large implants tend to ripple more long term than smaller ones because the heaviness can cause tissue thinning over time.

If you go under the muscle to decrease the chance of rippling, then the chance that your breast will deform and distort from muscle contractions during competitions must be taken into consideration.

At the end of the day, do you want to be big? Or do you want to be discrete? What does your surgeon say? You could choose something in between.

Breast implant size and volume is based on your breast and chest wall measurements. If your anatomy can accomodate both, go with the bigger ones. Most women say I wish I would've gone bigger NOT smaller.  Realistically, there is only a 79cc difference in volume which is less than 1/2 cup size! Good luck.