There are a lot of factors that can lead to bottoming out after breast enlargement. We will assume that the implants are placed properly to start with, so that this is not a problem. Thus, bottoming out occurs when the lower aspect of the pocket expands allowing the breast implants to move lower on the chest wall. The issues at play here are your body, and implants themselves.
In terms of your body, the strength of your own tissues will affect how the implants change the lower pole of your breast. Some people will have a greater tendency towards stretching or expansion of the lower pocket, and this is related to the inherent characteristics of their tissues. In some women, there a very loose plane beneath the pectoralis muscles that can allow implants to continue to stretch or open up this area. This allows the implants to slip further down , and bottom out. Some people feel that implants on top of the muscles have less of a tendency to bottom out since the connections of the tissues in this plane are better defined than beneath the muscles.
In terms of the implants, the things that will lead to bottoming our are: larger implants, lack of good support of your breasts/implants with your bras, and saline implants rather than silicone. Larger implants weight more and will have a greater tendency to stretch the tissues. Likewise, I believe that silicone implants (since they are a cohesive gel) place lesser stresses on the tissues than saline implants, and tend to bottom out less.
Sporting activities like running can affect the shape of your breasts if you are not wearing good support in your bra. Actually, there is sports bra I recommend to my breast augmentation patients. It's the "Endure" bra made by Under Armour. This bra gives fabulous support and is a great investment to protect the longevity of your result.
The implants you are looking at are not excessively large in terms of volume. So, go ahead and run, but get a good sports bra (i.e. the Endure).
Good luck,
David B.