Would upgrading from 325cc to 500cc saline implants be too much for my petite/athletic frame? (photo)

At size 36 each 200 cc’s of implant corresponds to 1 cup
size change. If the original implants
did not satisfy your requirements, it is unlikely that additional 175 cc’s
will. I recommend a new technique called
The Mini Ultimate Breast LiftTM.
A circumareola approach is used to reshape your breast tissue, displace the
areolas medial and enlarge the implant pocket medially. Implants alone will not solve your problems. Best Wishes,Gary Horndeski, M.D.

It is possible to #increase your implant size with a #revision. However, the #pocket will need adjustment and a #breast lift may be necessary; to better #support the new implant. Additionally, a mesh for further support may be helpful depending on the #implant size and skin #elasticity.Depending on the reason for the breast revision determines the sizing outcome. However, there are no guarantees with size predictions for revisions. Initially, if a woman wears a size 36 AB bra and would like to be a C cup, which is the average size that women wear and the average size bra that is sold, she needs a volume of approximately 350 to 390 cc to make that change. This becomes less if she's wearing a 34 bra, because it has a smaller strap size and the cups have a smaller diameter. A smaller diameter implant would then be a volume of approximately 300 to 350 cc. In the same way if she was more petite, maybe a double zero or size one and wore a 32 AB bra, then to become a 32 C cup she would only require about 250 to 300 cc. This is a general number, and of course there are other factors that will affect this. 
Other factors which contribute to the outcome are #size, #shape, and #profile.  The #profile is determined by the amount of #projection versus roundness that the you desire, as well as the #size of the implant and size of your chest. Also, the placement or incision location will also have an effect on profile. Another element to a satisfying result of your #augmentation is realistic #expectations.#Shaped implants are all #textured and should not rotate when used properly. In my practice, about 20% of the patients benefit from shaped implants. 
You will learn if you are a candidate to increase implant size. and more about the #options best suitable for you, during a one-on-one in-person #consultation.

Simply stated, 500 cc implants are very large for a petite athletic woman. In this situation, long term problems such as thinning, sagging, and visible ripples are more likely. The closeness of the cleavage relates not just to implant dimensions but the attachment of the pectoral muscle and releasing the muscle to make room can cause symmastia.

can always be done if you're willing to accept the risks and potential long term complications such as thinning of your skin.  So I always tell my patients to live the life with their current results and see if they really need the increase in size.  Your results look good and I hope you know that.  A bra should easily create cleavage by pushing breasts together.  It comes down to costs and risks...

Your photo already shows some roundness in the upper breast which suggests your implant is not submuscular, or is hitting the limit of what might cover and blend with your breast tissue. The 500 will fit, though think carefully about how natural you wish to be.

You seem to have a great result. Larger saline implants are likely to result in visible rippling. Creating more cleavage by moving the implants to the center may make your nipples point sideways.