Scale the node boxes when there are lots of them

The first shrinkage happens when there are 25 nodes, and it makes the
boxes half the size (4 times smaller area-wise). The next one is at
100 nodes (when they have roughly the same area as the first one), and
makes them 1/4 the size (16 times smaller area-wise). The next step would
happen at 400, but then they would be too small.

Change-Id: I414bd8724e12891228c81e7f71549d08ae207753