Repeated travel back and forth between the overworld and the nether results in increasing memory allocated to tenured space.

I used these java flags: -XX:+PrintTenuringDistribution -XX:+PrintCommandLineFlags -XX:+PrintHeapAtGC -XX:+PrintGCDetails -Xloggc:GC.log -XX:+PrintGCDateStamps

and a "tail -F GC.log"

Start by loading up the overworld, and traveling around a little. Then, make a nether portal, and go back-and-forth.

The travel around the overworld let the memory get to an initial baseline.
Repeated travel into the nether keeps raising the tenured space.