When you go migrate your Java application from 32-bit to 64-bit JVM, the heap requirement suddenly increases, almost double, due to increasing size of ordinary object pointer from 32 bit to 64 bit. This also adversely affect how much data you can keep in CPU cache, which is much smaller than memory. Since main motivation for moving to 64-bit JVM is to specify large heap size, you can save some memory by using compressed OOP. By using -XX:+UseCompressedOops, JVM uses 32-bit OOP instead of 64-bit OOP.
