Heap size doesn't go back to normal after loading

I'm loading 200G of data to a 2 node cluster with 15G of heap size on each node.
After loading has finished I notice that the heap usage stays on 10G and executes small size garbage collections.

If I restart the nodes, both go back to 5G usage and status is green.
My question is why do I have to restart the nodes so the heap size goes back to normal. I would expect it to go back to 5G on its own, because I still need a large heap size free for filter cache and more loading to come..

Anyone knows if this is a configuration issue?

*using Elasticsearch version: 1.4.1

Thanks in advance