What is the effect of the implant weight on the breast tissue (comfort, longevity of results)?

Increased weight equals increased gravitational pull, which increases stress and strain on the breast and surrounding tissue. This can ultimately lead to sagging, as well as cause back and neck pain.
Breast movement during activity exacerbates the effect of weight which can cause discomfort and increase the stress and strain on the breast tissue.  
Less weight equals less strain. It’s that simple.