The apparent growth rate of breast tissue does slow down over time. It starts out gratifyingly fast, but tapers off. You are probably growing the same number of cells per day. But as the volume gets bigger, the change in diameter will be less noticeable.
For math nerds, the diameter should grow as the cube root of the volume. I don't know how exact that relationship is, but it seems to be close for me.
So it could be that you are just further along, and the apparent growth rate may just be slower than it was originally.