Due to dynamic resizing of heap for old gen, at one stage it reaches to consuming most of memory even from eden space. Now, when something needs to be allocated to eden, heap gets resized again. And this becomes the new normal, where heap gets resized repeatedly resulting in a huge performance cost to the application.
This issue is addressed on stack overflow - http://stackoverflow.com/questions/41488024/hotspot-jvm-g1gc-heap-resizing-issue