If the heap is under 32GB, the JVM can use "compressed oops" with 4 byte "pointers" to 8 byte offsets inside the 32GB heap instead of 8 byte pointers. This makes everything more compact. Since Java doesn't have structs, everything is a pointer and arrays of things are arrays of pointers. So a 48GB heap won't fit a lot more data than a 32GB heap.

See

https://shipilev.net/jvm/anatomy-quarks/23-compressed-refere...

https://wiki.openjdk.java.net/display/HotSpot/CompressedOops